On the Myarian Pelecypods of Japan : Part II. Geological and Geographical Distribution of Fossil and Recent Species, Title Genus Mya Author(s) Fujie, Tsutomu Citation Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy, 11(3), 399-429 Issue Date 1962-03 Doc URL http://hdl.handle.net/2115/35932 Type bulletin (article) File Information 11(3)_399-430.pdf Instructions for use Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP ON THE MYARIAN PELECYPODS OF JAPAN Part II. Geological and geographical distribution of fossil and recent species, genus Adya. By TsutQmu FUJIE Contribution from the Department of Geology and Mineralogy, Faculty of Seience, Hokkaido University, Sapporo ; No. 861 Contents I. Introduction ............................. 399 II. Taxonomicai problem conceming the species, Adya j'mponica and Mya 7mpontca oonogtzz . , . , . 400 III. Geological and geographical distribution (off Japanese islands and near- adjacentlands) ............................401 IV. Geological and geograpnical distribution in the Northern Hemisphere . 409 V. Localities for fossil and recent Myarian species . 414 Acknowledgments . 421 References ....,.............,..........421 I. Introduction Mora, the pelecypod genus is very widely distributed in the seas of the Northern Hemisphere. In many places myarian species are 1<nown in general as sea food. The species belonging to the genus Mora are generally observed to live in very shaliow seas, especially near the tidal zone. They are assumed to be we!i adapted to somewhat brackish-polyhalinic water. In general they are endobiotic, burrowing sub-upright burrows to a depth of some 15 to 30cm in the sea bottom. Hitherto, numerous species have been proposed by various scientists both in tespect to fossil and recent forms. Their oldest appearance is dated back to the eariy Palaeogene. As their stenobathic, and stenohalinic nature in ecological conditiQn, besides their special habitat df burrowing life is clearly known, myarian fossils can be assumed to give also good keys for the solution of various problems concerning palaeoecology. Within such a wide scope, the author has long continued to study the myarian fossils of Japanese Tertiary ingiuding recent forms. In a former report (FuJiE, 1957) fossi! and recent species have already been described in detaii. In this paper, various problems in respect to the geological and geographical Part I. Summary of the Study of the Genus Adya from Hokkaido; Jour. Fac. Sci., Hoklgaido Univ., Series IV, Vol. 9, No. 4. distribution of m,yatian species will 'be 'briefly presented. II. Taxonomical problem concerning the species, Mya jcoponica and MJyw ,iop)oinde" oonoyai As far as the present author is concerned, there seems to be much con- fusion in the taxonomy of recent species of Mya. The problem lies in whether or not there reaily exist one or more species besides Mova trztncata and A4Zya arenaria. Since 1856, thet'e have been variously proposed new specific names. For instance, Mya interniedia (DALL, 1898) and A{lya 7'aponica (JAy, 1856) were proposed for the Pacific form, while Adya truncata fornza owata (JENsEN, 1901) and Mya 2bseudbarenaria (ScHLEscA, 1931) were also described respectively as names for the Atlantic form. However there have remained ambiguities in the description, and illustrations when their specific names were first proposed, preventing any exact comparison with each other as well as with other known species. MAcGNiTiE, N. (1959) after detailed study recently expre ssed the view that Adya truncata fo7vna owata and M. intermeclia are entirely and certainly synonymous with Mbla pseucloaienaria. Further he regarded a certain species having been hitherto called Adya j'mponica, to be conspecific with Mora arencu"ia. According to N. MAcGNiTiE, the following three specific names are accor- dingly valid for the living myarians: Mbla trtmcata L. 1758, A4. ar'enai'ia L. 1758 and M. Pseucloarenai'ia (ScHLEscH) 1931. The author, however believes in the specific vaiidity of Mya J'mponica. The specimens collected from the sea of Tokyo Bay, and described as well as illustrated by N. MAcGNiTiE, (p. 187, pl. 19, fig. 8, 1959) are nothing but Mya japonica oonagai, In this regard, the author is perfectly in agreement with his views. The specimens of Tokyo Bay, just mentioned, are however quite distinct from the myarian species now living in "Volcano Bay" (=Funka wan) in southwestern Hol<kaido, facing the Pacific, from which JAy (1856) first described Mova j' mponica. There are indeed two species now living in Volcano bay. One of them is easily identifiable as Mora j' aponica oonagai while the other one is clearly distinct from Adya j'ciponica oonogai 'in having thicker test, chalky grey in colour, and larger and stronger chondrophore of the left valve. JA,y paid special attention to the three points above enumerated, when he proposed the new name ` Vaponica" for the Japanese myarian form. The author also regards the above stated three criteria to be well justifying the specific distinction be- tween Mbla j' aponica oonagai and Adi 1'aponica. Besides, the author has come to the conclusion that Mbla pseuclbarenaria is entirely conspecific with Mya 7'(iponica, after careful study. The synonymity between Mova pseudoarenaria and M. intei-media, Al truncata fo7"ma ovata is also indubitable, as MAcGNiTiE has already stated. On the Myarian pelecypods ef Japan 401 Another troublesome problem is in the taxonomic ambiguity concerning Mbla 1'aponica oonagai and M. a7enaria. As the author said in a former paper (p. 406, 1957), the synonymity seems almost indubitable between Adya arenaria, and the Japanese species Mora 7'cipo- nica oonqg'ai. The latter species now lives in the sea around the Japanese islands ; besides it has been not seldom found and described from the Miocene to Pleistocene deposits in the islands of Hokkaido and Honshu. As it has been repeatedly stated, all forms have been called Mora 1' aponica oonagai, while indeed theY closely resemble Mora arenaria. The author has, however, unfortunately had no opportunity to examine and directly compare shells of Mbla arenaria living now in the Atlantic, with the Japanese species Mya 1'aponica oonag'ai. So he could not but help hesitating to call the Japanese species under the name Mora arenaria. It is therefore with hesitation that the author treats the Japanese species, called Mora 7'aponica oonagai, under such circumstances. Therefore, the living species assignable into the genus Adya can be listed in the following : Mcra truncata L. 1758 M ai"enaria L. 1758 (== M j'mponica oonagai) M 1'ciponica JAy 1856 (:=: M. inte7Amedia, M trt{ncata .forma ovata and M pset{clba7"enaria) It is accepted that the genotype of the genus A42ya is Mbla truncata L. possessing shells posteriorly truncated, from which most fossils and living species greatly differ in respect to shell form. WiNcKwoRTH (1930), proposed accord- ingly the subgeneric name C`Arenomya" for most species having shells to be posteriorly narrow, with rounded margin in distinguishment from Adya truncata in subgeneric name. MAKiyAMA (1934) and HABE (1952) accepted this classi- fication. As is recognized in general however, myarian shells individually show marked variations in their outer form, and the author believes now, such classi- fication as the above to be not always tenabie. If the genus Adya must be divided into groups, it would be better classified into the following two species groups, based on the two distinct types of chondro- phore, which the author described in detail in the former paper: the first group includes Mora cuneijb7vnis, M trzencata and Ml japonica whilst the second group consists of Mova e2oensis, A4. grezvingki and Al 7'ciponica oonagai. III. Geological and geographical distribution (off Japanese islands and near.adjacent lands) After long and continuous study, the geographical distribution for myarian species of Japan has become recently clear in detail to the writer, based on his specific identification of the specimens stored at the Department of Geology and Mineralogy, Hokkaido University including the author's own material. AIso description on myarian species, have been re-examined both in respect to fossil and recent forms, The results of such study are incorporation the accompanying maps. (Figs. 1 to 6) Before going into an explanation of these maps, the author however believes it necessary to mention the following. First of all, the geological age is always of very long duration. Secondly, as fossils are only to be found on outcrops or in boring cores, so the precise extent of distribution of fossi}s is not always easy to estimate. The formations with myarian fossils are entirely covered with thick overlying rocks. Accordingly it is hard to learn the true distribution of the fossils. That is to say, there may be always left the possibility that the Eoce"e P47 $EA 5AFMCHAi<(l/bPe"' OKMOr$K EI)9 ,,･ SKH LIN Vo :. p Mya eioensis ･lsls. KURIt 2 km o 500 /. PRi-MORSKAY ptrov. 2 2./5tSopSO HO 1DO . o 1 "or dv $SA OF it A rw eq JAP o k o 35' S<ORE . 'V:t2Sift{S-il HONSHU o p . N Y. paEu k if"p N Q K, U 3D . p Q oO Fig. 1. Geographical disttibution of fossil specles A{fya in Eocene age. On the Myarian pelecypods of Japan 403 true extent of distribution is greater than that area observed. Thirdly shells are neariy always deposited after transportation from the original places of inhabitati6n of the animals, through long or short distance. Thus their place of deposition does not always show the place of inhabition. Besides attention must also be paid to the fact that in the case of recent myarian species they sometimes became iocally extinct ip as short a period as half a century; on the other hand, some are newly migrated, or may have been artifically transported from farm to farm for culture.