n.Ass i 



PELECYPODA 353 



open mantle and sub-equal adductors would continue to be of a rounded and 

 symmetrical shape; while those which tended to produce elongate siphons, or 

 in which marked inequality of the adductors was developed, would probably 

 present more elongate or triangular outlines. The differences of form would 

 necessarily react upon the developing hinge, from the inevitable operation of 

 physical laws, and thus tend to produce in connection with particular lines of 

 evolution of form, particular types of hin^r. 



The recent researches of Bernard and Simroth have developed the fact 

 that in some Pelecypoda the ctenidium originates as a lateral plate, which 

 becomes transversely folded, and in which the reticulate form results from 

 subsequent perforations between the folds, and not from the concrescence of 

 originally separate filaments, as has been hitherto supposed to be the invari- 

 able mode. 



Neumayr has shown that, among the Palaeoconcha, ribbing existed in 

 various species along the dorsal as well as the other margin, and that it pro- 

 duced denticulations there ; and that when these denticulations had become 

 a fixed specific character, the ribbing disappeared from the area above the 

 hinge margin. 



In this way (as analogically in the recent Crenella et al.) the initiation of 

 the processes called hinge teeth began. Such projections, interlocking at a 

 time when the serrations of the other margin of the open valves could be of 

 little assistance in securing rigidity, offered a means of defence of the greatest 

 importance when fully developed by natural selection, one which would be 

 useful at every stage of development, but would increase in usefulness with 

 increase in size. In fact, this was just such a feature as would lend itself to 

 the fullest operation of natural selection. Once well initiated, its progress 

 was inevitable, and its variety and complexity only a question of time. 



Very recent studies by Bernard of the development of hinge teeth in 

 nepionic Pelecypods of many groups, show that in most if not all Priono- 

 desmacea and some Teleodesmacea there is first developed on each side of the 

 ligament (or behind it in Ostrea] a series of transverse denticulations or 

 minute taxodont teeth, forming what has been called a provinculum or primi- 

 tive hinge, independent (so far as yet observed) of the permanent dentition. 

 The latter begins subsequently by the development of distinct laminae on 

 the hinge plate. In the Teleodesmacea, toward the ends of the hinge plate 

 the primitive lamellae originate below the provinculum or in its absence, and 

 grow proximally. The inner ends of the anterior lamellae become hooked, 

 and these hooks separate from the distal portions which remain to form the 

 anterior laterals, while the hooks develop into the cardinals, and the posterior 

 lamellae into the posterior lateral teeth. The facts point, of course, to the 

 provinculum as representative of the primitive hinge as observed in many 

 Palaeoconchs ; but the gap between the provinculum and the beginnings of 

 the permanent dentition, indicates a suppression of certain developmental 

 stages which only further researches can supply. 



The dynamical origin of the shelly processes of the hinge, which we call 

 teeth, has only recently attracted attention. In this work Neumayr led the 

 way, and his contributions have been most valuable ; yet, as often happens 

 with pioneer work, he failed to grasp fully all the details of the subject, and 

 the nomenclature he proposed has required revision. Several groups or 

 kinds of teeth can easily be distinguished. These are not necessarily funda- 

 VOL. I 2 A 



