78 MOLLUSCA FROM THE GREAT OOLITE. 



anterior side, narrow, posterior side wider and more compressed, upper and lower margins 

 straight and nearly parallel, concentric plications few and delicate. 



A species unusually elongated and compressed posteriorly; it appears to be destitute 

 of radiating lines, judging from three examples which are the whole that have passed under 

 our notice. 



Height, 8 lines; length, 21 lines; diameter through both the valves, 6 lines. 



Localities. Minchinhampton Common, in the Great Oolite; it occurs also in the 

 Inferior Oolite of the same district. 



Pachyrisaia, Morris and Lycett, 1850. 

 Pachyrisma, Deshayes, 1851. 



Testa oblongd, cordiformi, cequivalm, valde incequilaterali, crassissimd, Iceviusculd aut 

 concentrice striata; umbonibas magnis angulatis contiguis et involutis, antice recurvis ; 

 carina obtusd, dorsali, posticd; ligamento externa, crasso, subelliptico, umbones versus 

 bifurcato. Dente et fovea cardinali unicd in utrdque valvd; dente magno, obtuso, irregu- 

 lariter conico later ibus compressis, et dente parvo accessor io in valvd dextrd ; impression- 

 ibus muscularibus duabus ; posticd obliqud in lamina interna sitd ; anticd oblongd excavatd 

 processu dentiformi superne instructd. 



Shell oblong, cordiform, equivalve, very inequilateral, thick, with large, angulated, 

 contiguous, and involute umbones diverging anteriorly ; an obtuse-angled posterior dorsal 

 keel divides the surface into two portions ; ligament large, external, somewhat elliptical 

 and bifurcated towards the umbones, to the apices of which a groove passes for its recep- 

 tion, as in Isocardia. Hinge massive, consisting of a single large, obtuse, conical tooth 

 m each valve, compressed laterally ; and a pit by the side of it to receive the corresponding 

 cardinal tooth of the other valve ; the right valve has, in addition, a small accessory tooth 

 placed upon the anterior margin of the cardinal pit. Muscular impressions two, the pos- 

 terior one is supported upon a raised projecting plate, which descends from beneath the 

 hinge obliquely backwards, the position of which is marked upon the external surface by a 

 slight furrow ; anterior impression deeply excavated, of an oblong form, and with a small 

 tooth upon its upper margin. 



This genus has some affinities with Isocardia, Opis, and Megalodon, the latter of which 

 it appears to represent in the Jurassic period, and with it may constitute a family 

 " Megalonidm" It is distinguished from Megalodon by the cardinal tooth in the right 

 valve not having been divided as in the latter genus. Megalodon has the anterior mus- 

 cular impression upon a somewhat raised or lamelliform plate ; but the posterior raised 

 plate of Megalodon presents a near approximation to that of Pachyrisma. From Opis it is 

 sufficiently distinguished by the characters of the dentition. The dichotomous ligament 

 resembles that of Isocardia, and when viewed anteriorly, it reminds us of the recent 

 Isocardia cor., with its large and graceful diverging umbones. Pachyrisma, then, may be 



