84 PALAEONTOLOGY OF THE U P P E II MISSOURI. 



angular. Surface ornamented by small, simple, obscure concentric eostre, which on the posterior side of the valves 

 descend at first perpendicularly, after which they are deflected forward parallel to the basal and anterior borders. 

 Length and height, each about 0.97 inch ; convexity, 0.58 inch. 



The specimens of this species we have seen, are not in a condition to have re- 

 tained fine surface markings if there were any ; nor do they show very satisfactorily 

 the character of the corselet, though it seems to have been marked by obscure 

 radiating costse, and is bounded on each side by the distinctly angular umbonal 

 slopes. The specific name was given in honor of Mr. T. A. Conrad, the well-known 

 Palaeontologist of Philadelphia. 



Locality and position. Southwest base of the Black Hills ; in the lower Jurassic 

 beds of that region. (Type No. 212.) 



FAMILY MYTILID^E. 



Shell inequivalve, inequilateral, closed, elongate, oval or oblong; 

 covered with a thick dark epidermis ; interior more or less pearly ; liga- 

 ment internal or submarginal, very long; hinge nearly or quite edentu- 

 lous, or sometimes crenate. Posterior muscular impression large, and 

 faintly marked ; anterior generally small. Pallial line simple. 



Animal with mantle margins free, or united behind so as to form a 

 more or less complete anal tube ; labial palpi elongated, pointed, and 

 free ; gills two on each side, elongated, nearly equal, united to each other 

 behind, and to the mantle. Foot cylindrical, grooved, and byssiferous. 



This group includes the following genera, viz.: Mytilus, Volsella, Pachymya, 

 'Litliopliagus, Myrina, Adula, Crenella, Hippagus, 1 and Stalaffmium? 



Messrs. H. and A. Adams divide it into the following subfamilies, viz. : 



1. Mytilinse. Hinder part of mantle but slightly produced ; anterior muscular scar generally small. 



Including Mytilus, and Myrina. 



2. Crenellinse. Hinder part of mantle produced so as to form false siphons. 



Includes Crenella, Volsella, and Adula. 



3. Lithophaginse, Hinder part of mantle more or less produced ; anterior adductor muscle moderate. 



Includes Lithophayus. 



The fossil genera Pacliymya, Modiolopsis, and a part of species referred to Ortlto- 

 nola, seem to belong to this family ; but as we know them only as extinct species, 

 it is scarcely possible to determine to what particular section of the group they 

 most properly belong. 



' Chenu (in Man. de Conch. II, p. 169) places Hippagus, Lea, in the family Trirjoniidee, and 

 figures Wood's sp. verticodius, as a cretaceous example of that genus. This, however, is far from 

 correct, that species being the type of the genus Veriicordia and a Miocene shell ; while Hippagus 

 isocardoides, Lea, a widely distinct form, from the Eocene, is the type of Hippagus, and belongs, as 

 we think, to the Mylilidse, very near the genus Crenella, if it is indeed even generically distinct. 



9 If Nucunella, and Nuculocardia, D'Orbigny, are distinct from Stalagmium, they should appa- 

 rently be placed at least near that group in the Mytilidse. Chenu, in the work above cited (p. 181), 

 places Nucunella in the Arcidse, and figures its type, N. Nystii, both there and on p. 153, under 

 Crenella, in the Mytilidse. 



