No. 1] Proc. Japan Acad., 63, Ser. B (1987) 5 2. Note on Pachydesmoceras, a Cretaceous Ammonite Genus By Tatsuro MATSUMOTO (Communicated by Teiichi KoBAYASHI, M. J. A., Jan. 12, 1987) The genus Pachydesmoceras is widespread in the Cretaceous deposits of various regions of the world. When Spath (1922) established this genus under the Puzosiinae of the Desmoceratidae, he designated the specimen of Desmoceras (Puzosia) denisonianum (Stoliczka) illustrated by Kossmat (1898, pl. 15, figs. 5a, b) from southern India as genotype. Since a genus should be established on a species and not on a particular specimen, the type species of Pachydesmoceras is Ammonites denisonianus Stoliczka, 1865. In this paper I give concisely a revised definition of P. deniso nianum ( Stoliczka) , with a result that the illustrated specimens of Kossmat represent another species. This is based on my study of the type specimens kept in the Geological Survey of India (GSI.), Calcutta, and supported also by subsequent collections from Japan and Madagascar. Palaeontological descriptions. 1. Pachydesmoceras denisonianum (Stoliczka) Lectotype (designated herein) :-GSI. 208, figured by Stoliczka, 1865 (pl. 66a), from the Ootatoor [Uttatur] Group, northeast of Odium, southern India. Diagnosis :-Shell large, with low to moderate ratio of whorl expansion and involution of 0.3 to 0.4; umbilicus moderate to fairly wide. Whorls thickly ovoid or subcircular in cross-section in more or less late growth-stages, with convex flanks and subrounded umbilical shoulder. Shell in earlier growth-stages similar to Puzosia in shape but provided with long ribs which arise at or near the umbilical rim; periodic constrictions or flares of normal frequency. From the middle growth-stage onward, the flares become frequent, turning to be major ribs, and the intervening several ribs of unequal length also coarsen; the ribs and flares simply concave on the flank and gently projected on the outer part. On the adult body-chamber, thick major ribs disposed at wide interval on the main part of the flank, broadened but lowered outward with gently concave curvature, fading away on the venter. Suture of the same pattern as that of Puzosi.a. Remarks:-In the original description of Ammonites denisonianus Stoliczka, 1865, four specimens (syntypes) were illustrated, which he thought as showing aspects of variation. Kossmat (1897-98) was right in excluding the twos from the four syntypes to Holcodiscus sparsicostatus Kossmat, 1897 and Pachydiscus jimboi Kossmat, 1898, although the former is Kossmaticeras and the latter Lewesiceras or rather Nowakites in the current generic assignment. The re- maining two syntypes are GSI. 208 and GSI. 265. The latter (Stoliczka, 1865, pl. 66, figs. 2, 2a) is doubtful (erroneous) in the locality record "northwest of Ootacod, Arrialoor Group" in the explanation of plate, but this may be the locality of Ammonites ootacodensis Stoliczka. The specimen is fairly well pre- 6 T. MATSUMOTO [Vol. 63(B), served but wholly septate and the body-chamber characters are not observable. Therefore, I designate the remaining last specimen, GSI. 208, as the lectotype. This is very large, slightly more than 1080 mm in diameter, and probably repre- sents an adult macroconch. Although its right side is unpreserved, its left side is well shown, with the body-chamber preserved for nearly 250°. Its dimensions (in mm) at the point 180° adorally from the last septum are as follows : diameter=995, width of umbilicus=410 (.41), whorl-height (H) =343 (.35), ratio in whorl-height increase per half whorl=1.44, whorl-breadth (B) presumed from the preserved half=300 (.30), B/H=0.87. The specimen described as P. denisoni by Collignon (1961, pl. 8, figs. 1, 1a), from the Cenomanian of Madagascar, is probably a macroconch of this species. Examples from Japan will be described later. 2. Pachydesmoceras kossmati sp. nov. Holotype :-One of the illustrated specimens of Kossmat (1898, pl. 15, figs. 5a, b ; pl. 14, fig. 6) under Puzosia denisoniana (Stoliczka), from the Uttatur Group (upper part?) of southern India. Paratypes :-Another illustrated small specimen of Kossmat (1898, pl. 14, figs. 5 a, b) is probably an immature example of this species, whereas there are Fig. 1. Pachydesmoceras kossmati sp. nov. A paratype (Kera's no. 398) from the Kaneobetsu, Oyubari area, central Hokkaido. Lateral (A) and ventral (B) views. Scale bar=30 mm. No. 1] Note on Pachydesmoceras 7 several larger specimens from the Turonian of Japan. One of them (no. 398 of M. Kera Coll.) is illustrated in this paper (Fig. 1). Diagnosis :-Microconch medium-sized ; macroconch large, both fairly in- volute and fairly narrowly umbilicate with rather high rate of whorl expansion. Whorls compressed in the young stage, with frequent constrictions and associated narrow flares, lirae on the shell surface and then riblets appearing on the outer part; gradually broaden in the middle growth-stage, thickly suboval in cross-section, with somewhat convex flanks and subrounded umbilical shoulder, and ornamented with long, major ribs, which develop from frequent flares, and several weaker, minor ribs of unequal length on each interspace of the major ribs. The ribs are nearly prorsiradiate or gently sigmoidal on the main part of the flank and gradually curved forward on the outer flank, crossing the venter with projection. Outer whorl of the adult stage slightly higher than broad or nearly as high as broad and rounded but broadest below the mid-flank, having strong and coarse ribs of unequal length, some of which may develop from the major ribs of the preceding stage. These ribs more or less dense, numbering 12 on the average per quarter whorl, and gently concave on the flank, crossing the venter with moderately convex curvature. Suture of the same pattern as that of Puzosia. Remarks :-The holotype and the specimen illustrated in this paper (Fig. 1) both have the body-chamber which shows the ribbing of adult stage, but the former is 173 mm and the latter about 330 mm in diameter of restored outline. Since the dimorphism is confirmed in certain other genera of the Puzosiinae, the former can be regarded as macroconch and the latter as macroconch, although the apertural margin is not preserved. P. kossmati differs from P. denisonianum in its higher rate of expansion and deeper involution of whorls and narrower umbilicus. In the former periodic flares are more frequent on inner whorls and coarse, alternating long and short ribs characterize the body-chamber. Although the two species are separated, they are good representatives of Pachydesmoceras. I should give somewhat comprehensive generic diagnosis, when more species are described. So far as the examined specimens are concerned, P. denisonianum occurs in the Upper Albian and Cenomanian, whereas P. kossmati from the lower half of the Turonian. Likewise, other species of this genus seem to have a limited stratigraphic range. For more details another paper is under preparation. Acknowledgements. I wish to thank Mr. M. V. A. Sastry, former Chief Palaeontologist of the Geological Survey of India, and Dr. Jacques Sornay, formerly of the Museum National d'Histoire Naturelle, Paris, for their kindness to help me in the study of the type specimens there in 1964 and 1965. Thanks are also due to Mr. Masatoshi Kera for the generosity to let me borrow his specimen for this study, to Professor Emeritus Teiichi Kobayashi, M. J. A., for warm encouragement, and to Dr. Masayuki Noda and Miss Yoshimi Tanigawa for their kindness in taking photographs and preparing the typescript. References Collignon, M. (1961) : Ann. Geol. Madag., 31, 1-115, pls. 1-32. 8 T. MATSUMOTO [Vol. 63(B), Kossmat, F. (1897-98) : Beitr. Palaeon. Geol. Osterr.-Ung., 11, 1-46 pls. 1-8 (1897) ; 89-152, pls. 14-19 (1898). Spath, L. F. (1922) : Trans. Roy. Soc. S. Afr., 10, 113-147, pls. 5-9. Stoliczka, F. (1865) : Mem. Geol. Surv. India, Palaeont. Indica, ser. 3, (6-9),107-154, pls. 55-80. .