400 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 8, 



a few valves of Pachyrisma ; but the nine-incli overlying and fossi- 

 liferous browner bed has altogether thinned out and become lost, the 

 upper and barren limestones reposing immediately upon the white 

 bed. "We may then venture upon the inference, that the causes of 

 whatever nature, which produced the cessation of marine deposits for 

 a period at this place, were likewise coincident with, and perhaps 

 conduced to, the extinction of this moUusk. The interval which 

 elapsed between the completion of the nine-inch brown bed and the 

 commencement of that next above, marks its upward limit, but the 

 associated univalves survived these changes and are met with occa- 

 sionally much higher in the series. 



The substance of the test of Pachyrisma is converted into crystal- 

 line carbonate of lime, and owing to the hardness of the investing 

 stone it is seldom that a specimen can be detached in an entire con- 

 dition ; in the white bed indeed it is impossible to do so, but the 

 browner bed fortunately, being more arenaceous, has softer portions 

 which yield to the knife, and have enabled us by dint of perseverance 

 and the unavoidable destruction of many fine specimens, to disclose 

 the characters of its interior in a satisfactory manner. 



Pachyrisma*, Morris and Lycett, 1850. 



Gen. Char. Testa oblonga, cordiformi, aequivalvi, valde inaequilaterali, crassissima, 

 laeviuscula aut concentrice striata ; umbonibus prominentibus, antice recurvis ; 

 carina obtusa, dorsali, postica ; ligamento extenio, crasso, subelliptico, umbones 

 versus bifurcate. Dente cardinali in utraque valva magna, obtusa, irregula- 

 riter conica, et dente parvo anteriore in valva dextra ; impressionibus muscu- 

 laribus duabus ; postica in lamina auriformi laevata et concava sita ; antica ob- 

 longa, excavata, processu dentiformi superne instructa. 



An oblong, cordiform, equivalve, inequilateral, thick shell, with 

 prominent recurved umbones, and an obtuse posterior dorsal keel ; 

 ligament large, external, somewhat elliptical, and bifurcated towards 

 the umbones. A large obtuse conical cardinal tooth in each valve 

 compressed laterally ; the right valve has a small accessory tooth 

 placed upon the anterior margin of the pit which receives the large 

 tooth of the other valve. Muscular impressions two ; the posterior 

 one supported upon a raised, projecting, and concave auriform plate ; 

 anterior impression large, deeply excavated, of an oblong form, and 

 with a small tooth upon its upper margin. 



This shell is remarkable for the projecting and solid character of 

 the hinge apparatus, together with its general massiveness. These 

 features are. in striking contrast with the attenuation of the posterior 

 side ; this latter portion is consequently seldom well-preserved, al- 

 though the internal elevated auriform processes must have contributed 

 to strengthen this part. The large tooth in each valve projects 

 nearly at a right angle with the plane of the shell, and is bordered by 

 a large and deep pit to receive the tooth of the opposite valve ; the 

 small dentiform processes, bordering the anterior muscular impres- 

 sions, are just in contact when the valves are closed ; that of the left 



* Traxvs, thick ; epeiafxa, support. 



