No. 89.] 



75 



lines of growth and fine obsolete radiating striae, which are usually more con- 

 spicuous in the exfoliated shell. The interior of the dorsal valve is marked by 

 the presence of a longitudinal septum, and the upper part of the ventral valve 

 by a deep sub-triangular muscular impression which unites with the rostral 

 cavity. 



Examples : Meristella IcRvis, M. hella and M. arcuata of the Lower Helder- 

 berg group; M. cylindrica, and M. oblata of the Niagara and Clinton groups. * 



Geological range of the species. In the Clinton and Niagara groups, the Lower 

 and Upper Helderberg and Hamilton groups, and probably in the Chemung group. 

 The carboniferous species of similar form, which I have examined, have a bilobed 

 muscular impression in the ventral valve, and correspond with those I have referred 

 to Athtris. 



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

 to Meristella, but marked by plications on the messial fold and sinus, and some- 

 times with obscure or discinct plications on the lateral portions of the shell. 



The internal structure appears to be the same as in Meristella, and the fine 

 obscure radiating striae and fine cancel lating 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 Rhtnchonella; but the plications are more 

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

 compressed than in Rhtxchonella proper. The internal structure appears, so 

 far as ascertained, to be the same as in Meristella. 



For these forms, I propose the generic name Leiorhtxchus. 



GENUS LEIORHYNCHUS (n.g.). 

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

 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 

 fibrous. Yalves articulating by teeth and sockets. Interior of ventral valve 

 with two short diverging dental lamellae, which extend into and are affixed to 

 the sides or bottom of the rostral cavity. The muscular impressions occupy a 

 narrow triangular cavity below the bases of the lamellae, and usually extend 

 about one-third the length of the shell. Dorsal valve with a well-defined medium 

 septum, which extends half the length of the shell : the hinge-plates are nar- 

 row, strong processes, embraced by the curving teeth of the opposite valve. In 

 numerous specimens examined, there is no evidence of internal spires; and 

 it is only the similarity of these forms to Meristella and Athtris, that affords 

 an argument in favor of the existence of these appendages. 

 The types of the genus are Atrypa quadricosta and j^. mesacostalis ( Hall, 

 Geol. Report of the Fourth District of New York ) = Leiorhynchus quadricosta 

 and L. multicosta described in this paper, from the shales of the Hamilton group. 

 For illustration of these genera, see end of descriptions of species of the Hamil- 

 ton group. 



