Dall — Hinge of Pelecypods and its Development. 461 



ered, or has given less weight to biological considerations not 

 imprinted on the shell ; while in my own case from the nature 

 of my previous studies I have been led to attack the problem 

 from the other side. Recent investigations, available only 

 since the date of JSTeumayr's paper, have thrown much light 

 on the inosculation of characters not before known to interlace. 

 Neumayr also, from my standpoint has insufficiently grasped 

 the importance of the different processes involved in the pro- 

 duction of the internal cartilage and its shelly coefficients on 

 the one hand and the denticnlation of the hinge margin on the 

 other. These two processes, though they must often have pro- 

 ceeded simultaneously in the same genus, were not necessarily 

 connected except in so far as by resulting stresses each might 

 react on the hinge-product of the other. So instead of having 

 a Desmodont type of hinge as opposed to a Prionodont, and, as 

 Neumayr would say, a Heterodont (Teleodont) type, we may 

 have either an Anodont (Paleoconch), a Prionodont (Taxodont), 

 or a Teleodont (Heterodont) type of hinge, either with or 

 without an internal cartilage and its accessories. 



By the elaboration of this view, as attempted in the fore- 

 going discussion, it seems to me the discrepancies so evident in 

 Neuraayr's system have been avoided ; the types of hinge 

 assigned their proper weight in the system ; while those 

 biological relations which are not fully reflected in the shelly 

 parts', have not been slighted ; though inevitably numerous im- 

 provements in detail will suggest themselves to students, or be 

 effected by a future expansion of our knowledge. 



As regards the Rudistes, if, as claimed by Woodward and 

 others, they possessed an internal cartilage, it is probable that 

 they must form a specially modified and extraordinary ramifi- 

 cation of the Chamacea. If, however, as is claimed by some 

 authors, there was no internal cartilage or external ligament 

 and the smaller valve simply rose and fell under the control of 

 adductor muscles guided by interlocking processes, it is evi- 

 dent, that this would establish an interrelation between the 

 valves unlike anything among the Pelecypods, and only com- 

 parable, perhaps, with that of certain operculated corals. In 

 the latter case the Rudistes would have to be regarded as rank- 

 ing at least among the subclasses, if mollusca at all. My own 

 impressions are that the first mentioned view is the more prob- 

 ably correct one. 



The opinion is occasionally expressed in scientific literature 

 that the shell is a " mere secretion of the mantle." This 

 usually proceeds from some person who- has not thoroughly 

 studied the molluscan shell, or appreciated its relations to the 

 animal. Such a statement is one of those half-truths which are 



