INSTITUTE OF SCIENCE, PHILADELPHIA. 519 



Lower Silurian to Devonian. 



Ex. Ambonychia, Amphicos/ia, Byssonychia, Byssopleria, Opisthoptera, Allonychia, Anoma- 

 lodonla, etc. 



The typical Ambonychia, according to Ulrich, is edentulous ; the forms 

 ordinarily passing under that name being now referred to Byssonychia. In 

 this group the byssus does not pass through a notch in one of the valves. 



FAMILY PINNID^. 



Shell mytiliform, not alate, dimyarian, the anterior adductor smaller; 

 equivalve, truncate and wholly open behind ; edentulous ; area linear ; liga- 

 ment parivincular, internal; shell-structure coarsely prismatic, with a thin, 

 partial nacreous lining; byssiferous; ventricle embracing the rectum, with 

 anterior and posterior aortas ; gills reticulate and plicate, with direct and re- 

 flected laminae, and free distal borders ; foot conical, elongate, grooved, with a 

 profuse silky byssus ; anal end of rectum free, erectile. 



Devonian to recent fauna. 

 Ex. Palatopinna, Aviculopinna, Pinna, Pinnigena, Alrina. 



The gills are erroneously stated to be filibranchiate and soldered to the 

 mantle, in a recent morphological introduction to the study of mollusks. 



FAMILY CONOCARDIID^E. 



Shell subtrigonal, anteriorly truncate and gaping, the margins of the gape 

 frequently produced into a tube-like rostrum and sharply serrate below, the 

 posterior end usually alate, the wing divided internally by a longitudinal ridge; 

 dimyarian, the anterior adductor scars smaller ; equivalve, more or less gaping 

 behind ; schizodont, with a single anterior lateral and an obscure or obsolete 

 cardinal tubercle ; area ill-defined, amphidetic ; ligament external, parivincular ; 

 shell structure cancellate or built up of hollow prisms, resembling those of 

 Pinna but not solid ; valves thick, internally marginate ; byssiferous ? marine. 



Silurian to the Carboniferous faunas. 

 Ex. Conocardium, Rhiphidocardium. 



These remarkable shells have been referred by most paleontologists to 

 the Cardiacea, with which they have no connection whatever except analogy 

 of form with a few, aberrant Cardiida ; an analogy, which does not offer any 

 point of real relationship. Carpenter pointed this out in his work on the 

 structure of shell, but with little effect on the literature. Neumayr was dis- 

 posed to regard them as related to Lunulicardinm, and it is probable that 

 some species of that heterogeneous assembly would more properly be placed 

 in this family. 



I do not feel positive that this is the final place for this family, but after 

 weighing all the facts have concluded that they would be less obviously out 

 of place here than elsewhere. It is entirely possible that this group may have 



