CONTRIBUTIONS TO PALAEONTOLOGY. 179 



racters of these are common to numerous well-marked forms in our Silurian strata 

 which show no evidence of the septum described, I advocate the restriction of the 

 generic designation to species of that type. At the same time, believing as I do that 

 the characters shown in the valves of Camarium are incompatible with an animal 

 like that inhabiting Merista, I feel compelled to advocate the separation of these 

 forms, and to maintain the Genus Camarium, adding, to the characters first given, 

 that it contains internal spires* as in Merista and Spirigera. 



With this restriction, the Merist?e proper consist of smooth, ovoid, circular or 

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

 corresponding wide mesial fold or elevation upon the dorsal valve. The hinge ar- 

 ticulation is not very different from that of Spirigera, to which they are allied; 

 but those which I have regarded as true Meristae have a deeply marked triangular 

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

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

 strong dental lamellae. This feature is not conspicuous in Spirigera : the dental 

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

 impression is different. The dorsal valve of Merista has a longitudinal median 

 septum; a feature which is obsolete, or partially obsolete, in the species of Spiri- 

 gera. The species of Camarium have the external form of Merista, but the wide 

 transverse arching septum in the ventral valve serves to distinguish it from Me- 

 rista or Spirigera. 



The Meristas 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. Camarium appears first in the 

 Lower Helderberg period, while Spirigera is known in a single species for the 

 first time in the Hamilton group. 



In the period of the Hamilton group, other new forms appear, apparently allied 

 to Merista, but marked by plications on the mesial fold and sinus, and sometimes 

 with obscure or distinct plications on the lateral portions of the shell. 



The internal structure appears to be the same as in Merista, and the fine ob- 

 scure radiating striae and fine cancellating concentric lines appear both upon the 

 surface and upon the exfoliated shell. In these forms the substance of the shell is 

 always thin, and the individuals are never so gibbous as in the species of the three 

 allied genera. 



Some of these forms approach Rhtnchoxella ; but the plications are more 

 rounded, and rarely or never continued to the lateral margins, which are more 

 compressed than in Rhtnchonella proper. The internal structure appears, so far 

 as ascertained, to be the same as in Merista. 



For these forms, I propose the generic name Meristella. 



^ GENUS MERISTELLA ( n. g.). i I - CPc '( > -A 



Shells variable in form, ovoid, circular or transverse : valves more or less equally 

 convex, with a median sinus upon the ventral valve and a corresponding eleva- 

 tion upon the dorsal valve; beaks imperforate, that of the ventral valve curving 

 over the smaller valve. Surface more or less strongly plicated; the mesial fold 

 and sinus always plicated, the lateral portions being sometimes nearly or quite 

 free from plications; concentrically marked by fine lines of growth and some 

 stronger imbricating lamellae. Substance of shell thin : structure distinctly 



• I have not eeen the original deeoription of this genus by Prof. Etmes. 



