INSTITUTE OF SCIENCE, PHILADELPHIA. 525 



ally tubular ; area, when present, amphidetic ; ligament amphidetic, alivincular ; 

 ventricle embracing the rectum, auricles intercommunicating below it, an 

 anterior and a posterior aorta ; gills filibranchiate, free, the filaments with or 

 without a reflected limb ; ganglia normal ; mantle lobes free, without siphons, 

 usually with papillae, ocelli, or other tactile prominences along the margin and 

 with an inner projecting lamina (curtain) near the margin, at right angles to 

 the plane of the valves ; pallial line simple ; foot small, usually subcylindrical. 

 grooved and byssiferous ; anal end of rectum usually free for a short distance ; 

 monoecious ; marine. 



Family PECTINID^. 



Shell inequivalve, inequilateral, auriculate, usually closed, monomyarian, 

 usually free ; area amphidetic or obscure ; ligament obsolete externally, the 

 immersed portion forming an internal resilium ; hinge-teeth taxodont in the 

 very young, obsolete later, the crural teeth feebly or not developed ; foot small, 

 the distal end often funicular or hood-like, modified for special uses ; byssus 

 feebly developed, sometimes absent ; soft parts generally as in the superfamily 

 diagnosis. 



Devonian to the recent fauna. 

 Ex. Plerinopeclen, Aviculopecten, Pecien, Pedum, Hinniies, Amusiutn, Chlamys, Vola, etc. 



The direct filaments of the gill are usually reflected in the shallow-water 

 species, but the abyssal forms like Aviusium sometimes have them simple and 

 direct only ; the foot is remarkably variable in form ; the byssus issues from a 

 gape between the valves {Ainusimii) or through a particular notch in the lower 

 valve, in the form of separate threads which are held apart and prevented from 

 twisting by a series of comb-like teeth (ctenolium), which are set on the ventral 

 edge of th2 sinus ; it is absent in many adult and some young abyssal species 

 and in the sessile forms. Most of the types are active, Himiites alone after an 

 active youth becomes sessile. In some species the edges of the mantle have 

 become slightly specialized for siphonal purposes, though the lobes are always 

 free. 



Family SPONDYLID^. 



Shell inequivalve, nearly equilateral, closed, pectiniform, obscurely auricu- 

 late, monomyarian, sessile ; area amphidetic, much larger on the attached valve ; 

 ligament alivincular, resilium more or less submerged ; byssus obsolete ; ar- 

 mature of the hinge taxodont, but obsolete in the adult and replaced by the 

 typically isodont development of the crura; gills filibranchiate with a reflected 

 limb ; foot small, subcylindrical with terminal funicle ; other anatomical char- 

 acters like PectinidcB. 



Trias to recent fauna. 

 Ex. Spondylus, Plicatula. 



The Osirea-like features of this group are due to the sessile habit, common 

 to both families. 



