'74 [Senate 



" possess this feature, the genus must be retained for the species with the shoe- 

 " lifter process."* 



It would appear, therefore, that the Genus Camarium, proposed by me in the 

 preceding Report, possesses characters identical with Merista as originally de- 

 scribed by SuESS, but which have been overlooked to some extent in consequence 

 of the reference to M. tumida as a typical form of the genus. This point will un- 

 doubtedly be best determined by Prof. Suess himself, and I shall not hesitate to 

 follow his views. 



At the same time, as the M. tumida of Dalman", an English and Swedish spe- 

 cies, in common with numerous well-marked forms in our Silurian and Devonian 

 strata, do not possess this feature, we can no longer, with propriety, refer them 

 to that genus. 



With this restriction, the Merist^ proper consist of smooth, ovoid, circular 

 or transverse shells, with usually a conspicuous sinus upon the ventral valve, and 

 a corresponding wide, often undefined, mesial fold or elevation upon the dorsal 

 valve; the hinge articulation being not very different from that of Athyris, to which 

 they are allied. 



The interior of the ventral valve, however, is strongly distinctive; and the sep- 

 tum or shoelifter process is not unfrequently shown in the cleavage of the beak of 

 that valve, in solid specimens, where the interior is inaccessible. 



The forms which I have regarded as Merista are similar to those above; but 

 instead of this septum, or shoelifter process, they have a deeply marked triangular 

 muscular area just below the rostral cavity of the ventral valve, which is border- 

 ed on the anterior side by a callosity of the shell, and on the two other sides by 

 the strong dental lamellae. This feature is not conspicuous inATHYRis : the dental 

 lamellae in that genus are shorter and less strong, and the form of the muscular 

 impression is different. The dorsal valve of those shells now under consideration 

 has a longitudinal median septum; a feature which is obsolete, or partially obso- 

 lete, in the species of Athtris. In the Oamarium, or Merista proper, the exte- 

 rior of the ventral valve sometimes shows what appear to be two diverging septa, 

 somewhat similar to those in the dorsal valve of Pen^tamerus, which are the 

 margins of the shoelifter process. 



The Meristidge begin their existence, so far as we know, in the rocks of the 

 Clinton group; and in this and the Niagara group there are several species, while 

 they are more numerous in the Lower Helderberg group : they occur likewise in 

 the Upper Helderberg rocks, and in the Hamilton group. Merista proper, so far 

 as we know, appears first in the Lower Helderberg period, while Athyris is known 

 in two species for the first time in the Hamilton group. 



Restricting, therefore, the signification of the Genus Merista to such forms as 

 were originally included by Prof Suess under that name, it becomes necessary to 

 designate those species of similar form, but without the peculiar appendage of the 

 ventral valve, by another generic term ; and I would therefore suggest the name 

 Meeistella, proposed by me last yearj. 



GENUS MERISTELLA ( n. g.). 



Shells variable in form, oval, ovoid, orbicular or transverse. Valves unequally 



convex, with or without a median fold and sinus; beak of the ventral valve 



apparently imperforate, incurved over the beak of the smaller valve; area none: 



valves articulating by teeth and sockets. Surface smooth, or with fine concentrif"- 



* Davidson, in letter to the writer. f Regents' Report on the State Cabinet, page 78. 



