INSTITUTE OF SCIENCE, PHILADELPHIA. 503 



as far as they have teeth, exception being made of the accessory lamellae, the 

 hinge is truly heterodont, and in its typical form probably owes nothing to 

 the influence of the internal cartilage. 



The Adductor Muscles. — The number and position of the adductors 

 was formerly accepted as a fundamental feature in classification. The results 

 of later researches have shown that there are many exceptions to any rule 

 which may be used for classification. The Dimyaria comprise at least one 

 monomyarian family {Tridacnidce), and among the Monomyaria the dimyarian 

 Dimya must find a place. MitlUria is dimyarian when young and monomy- 

 arian when adult. CJilamydoconcha has no adductors at all. For these rea- 

 sons an absolute foundation for classification cannot be afforded by the number 

 of the adductors, yet if allowance be made for degeneration caused by inequi- 

 laterality, torsion, and other causes, the general myarian types fall in fairly 

 well with the larger divisions based on the totality of characters. 



The Siphons. — In all Pelecypods there must be a path for water to ap- 

 proach the branchise and another for it to be discharged from the branchial 

 region without again passing over the gills. The /"i^/^firisw^r/zia; were, doubtless, 

 without siphons, but the necessity for the separation of the two currents is 

 such that the mantle was doubtless in some cases very soon modified to that 

 end. Forms evidently very similar to Leda, which has well-developed siphons, 

 appear very early in Geological time. The fouling of the water which ap- 

 proached the gills, by discharges from the rectum, or its deterioration by mix- 

 ture with that which had already by one passage over the gills been, to some 

 extent, deprived of its oxygen and food-contents, would obviously be a physio- 

 logical misfortune, and injurious to the mollusk, consequently various arrange- 

 ments for preventing this evil were soon developed. In some cases the man 

 tie-edge, though not actually connected, was sometimes so folded as to practi- 

 cally form two tubes, by one of which water was drawn in and by the other 

 discharged [Trigonia, Unid). Connections were finally established between the 

 partition-like folds of the edges of the opposite lobes of the mantle, and, in the 

 Naiades, these may, at present, be sometimes connected, sometimes discon- 

 nected, in the same species (Castalia, etc.). In various forms, especially the 

 burrowers, the tubes thus initiated were soon modified, developed and im- 

 proved. Long separate tubes enabled the effete products to be discharged in 

 a direction different from that by which fresh water was inhaled. Sensitive 

 papillse at the orifice of the incurrent siphon enabled the animal to avoid in- 

 draughts of mud or obnoxious substances. In species in which long siphons 

 were not developed, and in some siphonate forms, a partition or septum was 

 often produced which extended forward so as to practically separate the cavity 

 of the mantle into an anal and a branchial chamber (JPeriia, Lopliocardimn, 

 Aiiatind). But these changes were not respectively confined to the several 

 principal phyla of the Pelecypoda, but were more restricted in their scope. In 

 general among those forms which were most active {Pecten, Lima, Nucula, Trig- 



