BIVALVIA. 103 



resembles Pauopaea in the depression upon the middle or anterior side, and by the 

 presence of an occasional cardinal tooth in the hinge ; but these affinities are neutralized 

 by the differing figure of the anterior muscular impression, by the usually edentulous 

 hinge, and by the granulated surface. We regard, therefore, Myacites (Schlot.) as a 

 genus intermediate and connecting Pholadomya with Panopsea. 



The genus Myacites, Schlotheim, was founded upon certain Muschelkalk shells, which 

 belong to our great group of granulated Myadae, and have that kind of figure which 

 belongs to the Pleuromyae and perhaps to the Homomyae of Agassiz. Schlotheim, who had 

 no knowledge either of the hinge or of the test, characterised his genus in the following terms .- 

 " Testa transversa, incequilatera, subhians, olovatavelovalis, ventricosa l<svis, concentrice 

 striuta ; umbones anteriores." 



The meagreness and insufficiency of this description would render the genus valueless, 

 in the absence of other and more precise knowledge ; but as the Muschelkalk shells are 

 well known, Schlotheim has a claim to priority in the generic designation, and as the five 

 genera proposed by Agassiz must necessarily be referred to the same group, those of the 

 latter author must be discarded as superfluous. Our arrangement of the fossil Myadee 

 will be as follows : 



Pholadomya. 



Anatina, Sub-gen. Cercomya . . . Genus, Cercoraya, Ag. 



Goniomya . . Goniomya, Ag. . 



Ceromya . . . . Ceromya, Ag. 



Gresslya . . Gresslya, Ag. 



Thracia . . . Corimya, Ag. 



Myacites . Myopsis, Pleuromya, Platymya, Arcomya, and Homomya, Ag. 



GRESSLYA, Ag. 



Shell ovate, rather compressed, very inequilateral, sub-equivalve ; umbones anterior, 

 contiguous, compressed, acute, and incurved ; lunule excavated ; anterior side convex, its 

 border rounded; posterior side more attenuated, sometimes rostrated; superior border 

 rather convex, sloping obliquely downwards ; lower margin curved elliptically, borders of 

 the valves close, or with a very small posterior aperture ; ligament external, short ; hinge 

 line externally somewhat sinuous ; the shell is not perfectly equivalve, the unibo of the 

 right valve being a little higher in the other ; the test is extremely delicate, with fine 

 longitudinal plications, and with very densely arranged radiating rows of minute granules. 

 Hinge edentulous, but having an elongated lamina in each valve, that of the left valve 

 being inserted beneath the outer lamina of the other valve, as in a groove ; there is also 

 in the right valve an oblique internal rib, which extends posteriorly, and is only visible 



