502 TRANSACTIONS OF THE WAGNER FREE 



shell. Coincidently with its submergence the area of attachment must de- 

 scend from a position on the upper outer margin of the hinge-plate to one upon 

 the plate itself or excavated, so to speak, out of the mass of the plate. The 

 strengthening of the hinge-plate to bear the strains resulting from the new 

 position of the resilium, must proceed, pari passu, with the descent itself Thus 

 we find in the thin-shelled Anatinacea the pit, being reinforced by a special 

 thickening about it, assumes a spoon-like form projecting from the hinge- 

 plate, genei'ally supplemented by a special prop extending to the surface of the 

 valve, and also in many cases by the formation of an ossiculum which supports 

 the central and weaker portion of the cartilage or even takes its place. Among 

 the various names which have been proposed for this spoon-like process, that 

 of Chondrophore is perhaps the most generally used. It is a curious fact that 

 among the thin-shelled Anatinacea, owing to the sturdy resistance to the clos- 

 ing of the valves offered by the resilium and its accessories, and the dispropor- 

 tionate stress exerted by the adductors, an umbonal fracture just behind the 

 line of the chondrophore has become habitual {Thracia, Anatina). The dan- 

 gers which might threaten the animal from this breach of its defences seem to 

 be avoided by the deposition of conchioline along the fracture and doubtless 

 also by the deep-burrowing habit of these mollusks. 



Conversely, in heavy shells with a solid hinge-plate and thick umbones 

 adequate to the strains put upon them {Crassatellites, some Mactridce) no pro- 

 jecting chondrophore, prop or ossiculum is developed, but a flat surface or shal- 

 low pit is sufficient for all purposes. It should be remembered also that in 

 the few thick-shelled Anatinacea {^Euciroa, Entodesmci) no chondrophore is 

 developed, though the internal resilium is reinforced by a stout ossiculum. 

 It has been suggested by Neumayr and the writer that part of the armature of 

 the hinge, in the shape of teeth, is due to deposits made parallel to or induced 

 by the presence of the chondrophore and resilium. In another part of this 

 paper it is shown that those shelly plates, here named accessory lamella;, are 

 formed thus in the Mactracea, and there is some reason to think that the pres- 

 ence of the resilium in Pecten and Spondylus is not unconnected with those 

 changes of the auricular crura which lead to the assumption of dental func- 

 tions by the latter and to the obsolescence and even disappearance of the true 

 hinge-teeth. But it is well known that submergence of the resilium takes 

 place in various unrelated groups of bivalves, independently, and the writer 

 believes that most of these forms were furnished with teeth at the time these 

 changes were initiated ; and that these teeth, more or less modified or partially 

 displaced, st^l persist in the dentiferous forms; while the edentulous genera 

 have seldom developed any teeth which present the appearance of being due 

 solely to the presence of the chondrophore. 



The nearest approach to a hinge composed of dental laminae of such an 

 origin is found in Placiina, Placenta and Placnnanoinia, together with the 

 5/o«^//£/(2 already mentioned; while in the so-called Desmodonta o{'^&\xmAyr, 



