184 REPORT ON THE STATE CABINET. 



In the Chemung group, this type is feebly represented by some casts 

 of an undetermined species. 



The Meristella unisulcata of the Corniferous Umestone is referred to the 

 Subgenus 1 Pentagonia of Cozzens, and its varieties of form iUustrated. 

 The species is of rare occurrence in the Hamilton group. 



The reexamination of the shells of this group has developed some 

 farther knowledge of their internal structure. 



The general observations upon the Genus Meristella are as follows : 



The shells of this genus are oval, ovoid or suborbicular, elongate or 

 rarely transverse ; valves unequally convex, with or without a median 

 fold or sinus; beak of the ventral valve often with a circular foramen, 

 and incurved over the umbo of the dorsal valve. Area none ; valves 

 articulating by teeth and sockets ; surface smooth, or with fine concen- 

 tric lines of growth, and with very fine, indistinct or obsolete, radiating 

 stri». 



The interior of the dorsal valve is marked by the presence of a strong 

 hinge-plate or cardinal process ; and from the base of this proceeds a 

 thin longitudinal septum, which often extends for half the length of the 

 valve. 



The interior of the ventral valve shows a triangular fissure below the 

 beak, which joins a semi-circular perforation at the apex. At the base 

 of this fissure are two strong teeth, which extend in the thickened or 

 slender plates to the bottom of the cavity, and curve around the upper 

 part of the muscular area, which is broadly triangular or ovate. 



There is sometimes a thickening of the shell at the base of the rostral 

 cavity, which abruptly limits the muscular impression ; but there is 

 neither septum nor rudiment of one as in Merista. 



In well preserved specimens of M. haskinsi, where the apex is not too 

 closely incurved, the ventral beak has a circular foramen, and the tri- 

 angular space below, which is usually filled by the beak of the dorsal 

 valve, is closed by two deltidial pieces anchylosed in the centre. The 

 latter feature has been observed in M. barrisi, and probably existed in all 

 the species at some period of their growth. 



The study of the interior has shown that the thickened bases of the 

 crura extend forward for a short distance, or bend abruptly to the ventral 

 side, but recurving, descend into the cavity of the dorsal valve, follow- 

 ing its contour and that of the ventral valve in their succeeding volutions. 

 In the bottom of the dorsal valve, the lamellae, in the com-se of the first 



