NOTICE OF PALEONTOLOGY OF NEW-YORK, VOLUME IV. 155 



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

 an undetermined species. 



The Meristella unisulcata of the Corniferous limestone is referred to the 

 Subgenus? Pentagonia of Cozzens, audits varieties of form illuslrated. 

 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 concentric lines of 

 growth, and with very fine, indistinct or obsolete, radiating striae. 



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 tiie 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. haski?tsi, where the apex is not too 

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

 lar 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, following 

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

 the bottom of the dorsal valve, the lamellae, in the course of the first volu- 

 tion, are united by a loop which is produced by the extension of a slender 

 process from the band on each side, and these are united at a greater or 

 less distance from their origin. Beyond this junction the parts of the loop 

 again divide, and each one is produced in a curving band which arches for- 

 ward on the ventral side, arid thence returning is reunited to the sides of 

 the loop at or near the junction of the parts before noticed. 



These features are illustrated in the accompanying diagram; fig. 1 being 

 an oblique lateral view of the central portion of the spires, and fig. 2 a 

 view of the same parts from the dorsal side. 



