CLASS I PELECY PODA 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 hinge. 
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 e 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 
tee{h, 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 2A 
