INSTITUTE OF SCIENCE, PHILADELPHIA. 5II 



Class PELECYPODA. 



Aquatic, bilaterally symmetrical, acephalous mollusks, protected by a pair 

 of shelly valves secreted by the lateral portions of the mantle, connected by a 

 ligament, and moved by the contraction of muscles which connect the inner 

 faces of the valves ; feeding by ciliary action and destitute of a radula or jaw ; 

 breathing by lateral gills, the type of which is a midrib or stem, with a row of 

 transversely oriented leaflets or filaments depending from each side of the 

 stem, single, or mutually combined to form a direct or reflected plate; imper- 

 fectly sensible to light and rarely provided with peripheral visual organs ; pos- 

 sessing olfactory organs (osphradia), auditory and equilibrating organs (oto- 

 cysts), tactile papillae and a nervous system composed of (usually three princi- 

 pal pairs of) ganglia united by nerves, but without a pedovisceral commissure ; 

 provided with an extensile tactile or locomotor organ (foot) ; a closed, though 

 partly lacunary, circulatory system, containing (usually colorless) hsemo- 

 lymph, and operated by a single or paired cardiac ventricle and two auricles ; 

 a more or less convoluted intestinal canal, with its oral and anal extremities at 

 opposite ends of the body ; a stomach ; paired nephridia, connected with the 

 pericardium, and discharging independently of the rectum ; reproducing with- 

 out copulation, by eggs and spermatozoa; moncecious or dioecious; develop- 

 ment external to the ovary ; the post-larval young protected by a prodissoconch, 

 and sometimes exhibiting a nepionic stage ; with a distribution in geological 

 time from the Cambrian to the present day. 



The class appears to be subdivisible into the following groups, of which 

 the third represents the most perfected (though not always the most special- 

 ized) modern type of Pelecypod. 



There seems little reason to doubt that all these orders are descended 

 from a Prionodesmatic radical, or prototype, and that, for various reasons, the 

 first and second retain more evident traces of their origin than the third. 



Order PRIONODESMACEA. 



Pelecypods having the lobes of the mantle generally separated, or, when 

 caught together, with imperfectly developed siphons ; the soft parts in general, 

 diversely specialized for particular environments ; the shell structure nacreous 

 and prismatic, rarely porcellanous ; the dorsal area amphidetic or obscure, 

 rarely divided into lunule and escutcheon, and when so divided, having an 

 amphidetic ligament ; ligament variable, rarely opisthodetic ; armature of the 

 hinge characterized by a repetition of similar teeth upon the hinge-line, or by 

 amorphous schizodont dentition; habits active, sessile, or nestling, not burrow- 

 ing ; monoecious or dioecious. 



This group, originating with the earliest forms, has retained many archaic 

 features through immense periods of Geological time, developing from age to 



