458 Doll — Hinge of Pelecypods and its Development. 



little processes, the feeble remnants of the original fossettes. 

 An appendage analogous to and possibly homologous with an 

 original ossiculum has (that view being taken) revolved around 

 the cartilage, taken its place outside of the axis of motion of 

 the hinge, and instead of keeping the valves from crushing- 

 each other by checking the closing stress of the adductors 

 as in Verticordia or Bushia and other Anatinacea, it accom- 

 plishes the same end by locking over the reflected edges of the 

 shell on the dorsal surface acting, like the anterior adductors, 

 on the short instead of the long arm of the lever, and as before 

 in a sense opposed to the action of the adductors. Though 

 greatly specialized and modified this appendage retains some- 

 thing of the butterfly shape of a broad ossiculum. 



An appendage, sometimes called the styliform process or 

 apophysis, with its proximal end attached in the hollow of the 

 beaks, has been homologized by Deshayes with the cardinal 

 teeth. In Pholas costata it supports the posterior oral palpus 

 which is very massive, and some of the internal viscera. If 

 one of the umbonal laminse of Callocardia were detached from 

 its connection with the cardinal margin and allowed to project 

 into the cavity of the valve, it would somewhat resemble the 

 apophysis of Pholas. But on this view I am at a loss to 

 explain the present connections of this process about the 

 development of which little or nothing is known. How a 

 cardinal tooth should come to be situated inside the mass of 

 the body would seem to be hard to explain. The environment 

 of the Pholads is of a very special character and the modifica- 

 tions of the organization march with the peculiar circumstances 

 under which it exists. To enter into their mutual reactions 

 would take much space and obscure the more general questions 

 to which this paper is addressed. 



It may be added that in this order as well as the others the 

 particular constituency of each of the suborders, even the 

 number and scope of the families, must be regarded as tinged 

 with uncertainty from the magnitude of our ignorance. To 

 properly ascertain and correlate the data in regard to the differ- 

 ent genera and the families of which they are the members is 

 a labor worthy of devotion, but which will yet require a large 

 amount of original research. 



In the Prionodesmaeea the Nuculacea represent an archaic 

 type in many of their features. So far as the hinge is con- 

 cerned Area {N002 and related species) is perhaps the most 

 fully and typically developed instance of Prionodont dentition. 

 The JSfaiades declare in Spat/ia and Iridina their Prionodont 

 origin, traces of which are to be seen in the transverse stria- 

 tum of the teeth of many species of Unio, even when lateral 

 teeth have become well developed and preeminent. The same 



