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I.—Notes on Some British Palæozoic Crustacea Belonging To THE GEOLOGICAL MAGAZINE. No. C—OCTOBER, 1872. AETICLES. I.—NOTES ON SOME BRITISH PALEOZOIC CRUSTACEA BELONGING TO THE ORDER MEBOSTOMATA. By HENRY "WOODWAKD, F.G.S., F.Z.S.; of the British Museum. ' (PLATE X.) On the Genus Hemiaspis, H. Woodw., 1865.1 Species 1.—Hemisaspis limuloides, H. Woodw., PI. X., Figs. 1 and 2. When I first drew attention to this genus at the Bath Meeting of the British Association in 1864, only one nearly perfect speci- men was known. Mr. Salter was acquainted with this form, so long ago as 1857, and referred to it, among other new and undescribed Crustacea, in a paper "On some New Palaeozoic Star-fishes" found at Leintwardine, Shropshire,2 under the name of Limuloides. Portions of several others had also been met with, to which Mr. Salter attached MS. names in (the Museum of Practical Geology, Jermyn-street, but they have not been heretofore described. The most perfect of these Limuloid forms was described by me in a paper read before the Geological Society in June, 1865.8 (See Plate X. Fig. 1.) Since that date other fragments have been found, and also another nearly perfect example (obtained by the late Mr. Henry Wyatt-Edgell) of the form named by me Hemiaspis limuloides, which, having the upper central portion of the carapace preserved, nearly completes our knowledge of this species. (See PL X. Fig. 2.) The great interest attaching to this form arises from the fact that it offers just the desiderated link by which to connect the XIPHOSURA with the EUBYPTERIDA. Limuli, apparently differing but little as regards their carapace from the recent species now found living on the coasts of China, Japan, and the north-east coast of North America, occur as early as the deposition of the Solenhofen Limestone of Bavaria : and in the Coal-measures of England and Ireland several species of Bellinuri and Presticichice occur, in which behind the cephalic shield the body is composed of five more or less free thoracic segments, and the rudimentary abdomen, if not anchylosed in all, is so in most. (See Plate X. Figs. 8, 9, and 10.) 1 Extracted from the Author's Memoir on the Merostomata, Part iv. p 174 Pal. Soc. Mon.,T0l. for 1872. 2 See Ann. and Mag. Nat. Hist., 2nd series, vol. xx., 1857, p. 321. 3 See Quart. Journ. Geol. Soe., 1865, vol. xxi., p. 490, pi. xiv., fig. 7. VOL. IX.— NO. O. 28 434 H. Woodward—Notes on Palceozoic Crustacea. But in the specimen under consideration we have the eephalic, thoracic and abdominal divisions still remaining distinct, and ap- parently capable of separate flexure. This important character at once separates it from Limulug, Bellinurus, and JPre»twichia. I have 1 on this account not used the MS. name of Limuloides as a % generic appellationpp , but have proposepp d the name Hemiaspis (from 1 jfi, half, and dcnrh,dh a shield), reservini g thh e MSMS . name Limu-Li hides for the specific title of the most perfect species of the genus. (See Plate X., Figs. 1, 2.) But it will be observed that Hemiaspis is also, in general appearance, strongly severed from the other species of Eurypterida, as well as from the Xiphosura, in structure. The three divisions into head, thorax, and abdomen are more strongly marked. The abdomen is reduced to very slender pro- portions, less than one-third the length of the animal (the entire specimen measuring 2J inches in length by one inch in width). The carapace in general outline resembles Limulus, but is more dilated laterally. There is a small stellate ornamentation in the centre of each cheek, having five to six rays, and measuring about a line in extent; but whether this represents the position of the eyes I am quite unable to say. It is unlike the eye of any other member of the group, which causes me to doubt its relation to that organ. It seems more probable that the eyes were placed along the lateral margin of the glabella, not upon the centre of the cheek. There is a faint indication on one side of Fig. 1 and on Fig. 2 of a facial suture to the head-shield (as in the Trilobites), with a small aperture upon its border, which may -possibly indicate the true posi- tion of the eye, but it is by no means clearly defined. The surface of the glabella when perfect (as in Plate X) Fig. 2) appears to have been almost smooth,2 save that it is traversed by two ridges which, commencing as raised tubercles on the posterior border of the head-shield, three lines apart, gradually converge and unite, so as to form an arch, the summit of which nearly touches the front border of the glabella. Nine ray-like corrugations descend from the glabella towards the margin of the shield, and the whole surface of the carapace is very minutely granulated. The head-shield is armed, on each side, near the rounded posterior angles, with two principal spines directed backwards, whilst a fringe of lesser ones ornaments each lateral genal border. The thorax is composed of six strongly trilobed plates, the epimera being equal in breadth to the central portion of each segment. The first segment is the largest, being 1 line in depth and 1\ in breadth, including the epimera, which are pointed at their extremi- 1 With the concurrence of Mr. Salter given at the time. J In the original description of the glabella of Hemiaspis limuhides (see Quart. Journ. Geol Soc, 1865, vol. xxi. p. 490) I have described the glabella from a detached portion, " as ornamented with a semicircle of nine tubercles, and a tenth immediately •within the circle upon the elevated front, and two small tubercles at the posterior margin." The acquisition of the second specimen (Plate X. Fig. 2) proves this fragment to belong to another species, not to M. limuloides, as formerly supposed. H. Woodward—Notes on Paleeozoic Crustacea* 435. ties, and slightly overlap the folio-wing segment. Thefour following segments have the borders of their epimeral pieces rounded, and gradually decrease in breadth downwards from & lines to. 7, and increase in depth from $ line to 1 line. A section of one of the segments would present an outline like that of Phacops- among the Trilobites, namely, a triple corrugation. The 6th thoracic segment is more strongly arched than- the preced- ing ones, and the lateral borders are divided into two rounded lobes on each side; breadth 5 lines, depth 1 line, Th& abdomen consists of only three segments, each 2 lines in breadth, and 1^ line in depth., The first has no epimera, and appearpp s to move freely at its articulation with the last thoracic segment. The second and third segments have small epimeral pieces, which are bilobed, with, the posterior lobes more pointed. A line of small tubercles runs down the centre of these three- joints, which are somewhat raised at their articular borders. The telson is 12 lines in length' and li- line in breadth where it articulates with the abdomen; it tapers gradually to a fine point.. If we regard the first six body- rings from the head as thoracic^ and the remaining three segments, FMK, 1. —Semiaspis Iknaloides, H. Wootf- as abdominal, we must presume ward,L. Ludlow, Leintwardine. h, the head; th, the six thoracic segments; that each of these latter is a ab, the three abdominal somites; t, the telsoa. double segment, as compared with the segments of the Ewypterida proper. On the other hand, the presence of these three segments precludes our considering the head to be the cephalothorax and the succeeding segments the abdomen, a view controverted by me in my p^aper on the structure of the Xiphosura.1 The smallness of the abdomen, and its reduction from the assumed normal number of six to three segments, seems to indicate a form by which, with the help of others, we may bridge over the interval that has heretofore existed between these two groups, the Eurypterida and the Xiphosura. Although Hemiaspis is the only genus met with in Britain having this remarkable form, we know of three Eussian genera which pre- sent almost identical peculiarities of structure. Dr. J. Meszkowski has described two forms from the Upper Silurian of the Island of 1 See Quart. Journ. Geol. Soc, 1867, vol. xxiii. p. 28, plates i. and ii. 436 H. Woodward—Notes on PalcBozoic Crustacea, Oesel, namely, Tseudoniscus aculeatus (Woodcut, Fig. 2), and Exapinurus Schrenkii (Woodcut, Fig. 3) ; and Prof. Eichwald has described a third form under the name of Bunodes lunula (Woodcut, Fig. 4) from the same rich locality and formation.1 FIG. 2. FIG. 3. FIG. i. FIG. 2:—Pseudoniscus aculeatus, Nieezk. \ FIG. 3.—Exapinwus Schrenkii, Nieszk. [ All from the U. Silurian I. of Oesel, Baltic. FIG. 4.— Bunodes lunula," Eichw. ) All these forms show three well-marked divisions to their bodies, namely, head, thorax, and abdomen, and all (save Bunodes) possessed a telson, or tail-spine, and free articulated thracic somites. In addition to Hemiaspis limuloides, already described, there are certain other specimens in the Museum of Practical Geology, to which Mr. Salter has appended MS. names, namely— Hemiaspis (Limuloides) speratus, Salter, MS. „ „ optatus, „ „ „ tuberculatus, ,; In Lowry's chart of the genera of Fossil Crustacea designed by Mr. J. W. Salter and myself, Mr. Salter has figured a head-shield of Hemiaspis under the name of H. Salweyi. There can be no doubt that this form is identical with Limuloides tuberculatus of Salter. I consider his Limuloides speratus and L.
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