BIVALVIA. 79 



described as a Megalodon-like shell, the dental characters of which, however, are peculiar, 

 combined with the external figure of Opis and Isocardia. 



A detailed description of this genus and its affinities will be also found in the valuable 

 and useful work of Mons. G. P. Deshayes, the ' Traite Elementaire de Conchologie.' 



Pachyrisma grande, Li/cett. Tab. VIII, figs. 1 — 5. 



Pachyeisma grande, Morris and Lycett. Journal of the Geol. Society, 1850, p. 401. 

 Deshayes. Traite Elementaire de Couch., ii, p. 187, pi. 32dis, 

 f. 1—3. 



Testa cordatd, elongatd; carina obtusd, dorsali, posticd, latere antico brevi ; latere 

 postico profundi depresso ; striis n/onerosis, concentricis, irregularibus. 



Shell cordate, with an obtuse, prominent, posterior, dorsal keel ; posterior side deeply 

 excavated, with a mesial oblique furrow, forming with that of the other valve a cordiform 

 surface ; striae numerous, concentric, and irregular. 



In young specimens the form is less gibbous, the small dental processes are very 

 distinct, but the large tooth has little of the prominence which it afterwards attains, it not 

 having acquired the conical projecting form as in the adult state. 



The massive character of the hinge, umbones, and anterior side of the shell, presents a 

 striking contrast with the attenuation of the posterior side ; this latter portion is conse- 

 quently very rarely well preserved, although the internal projecting oblique plate must 

 have contributed to strengthen this part; the small dentiform processes bordering the 

 anterior muscular impression are just in contact when the valves are closed, that of the 

 left valve being received into a small depression above the corresponding process of the 

 right valve, the tooth of the right valve resting within the muscular impression of the 

 opposite one. The thickness of this portion of the test is such that in an individual 

 which measured six inches across, it was upwards of three quarters of an inch. 



Our shell nearly resembles a figure published by Catullo' of a cast of a shell named 

 Cardium triquetum, by Wolfen, 2 from the Jurassic strata of Antello, near Cardonino. The 

 shells figured by Pusch, (Polens. Palaeont., t. vii, figs. S, 9,) under the names Isocardia 

 exaltata, and I. ventricosa, have some affinity with our shell, and may belong to the same 

 genus. 



Geological position. This species occurs near to the base of a series of hard cream- 

 coloured limestone beds which extend from Minchiuhampton to Cirencester, the base line 

 of which has at one locality been ascertained to be forty-five feet above the Puller's earth ; 

 the position is therefore higher than the shelly weatherstones of Minchiuhampton Common, 

 and near to the middle of the formation. The limestones have, in the aggregate, a very 

 considerable thickness, but become browner and more sandy upwards. It is impossible 

 to disengage the crystalline tests from the hard limestone, but an accidental seam of softer 



1 'Saggio di Zoologia Fossile,' de T. A. Catullo, t. 1, f. d, e, f, f. 2 A; Padua, 1827. 

 - 'Abhandl. von Karnthenschen pfauenschen. Helmintholith.,' p. 48. 



