Doll — Hinge of Pelecypods and its Development. 459 



is true of Trigonia which has many points in common with 

 JSTaiades. To the latter immediately Mulleria bears such a 

 relation in its adult state as do the Monomyarian Pecten and 

 Ostrea to the rest of the Prionodesmacea as a whole. The 

 Prionodont character of the Mytilaeea will not be questioned. 

 Through them we pass to the Pectinacea, in which in Spondy- 

 lus we have the finest instance of a Prionodont hinge with few 

 teeth, as Area is of one with many teeth. The original trans- 

 verse grooving of the hinge is visible on the very young valves 

 of many species of Pecten, Janira, etc. The Ostracea are the 

 last term of specialization in this line ; the Anomiacea are 

 brought in by the total of their characters, though so far modi- 

 fied as to indicate little, by the hinge, of what I suppose to be 

 their origin. Above all it must be admitted that the Mono- 

 myaria and Heteromyaria represent not fundamental types 

 of structure but special modifications though geologically an- 

 cient. The presence of a prismatic layer of cretaceous otell 

 substance, outside of the pearly layer, is also characteristic of 

 most of the forms of this order. 



The remaining forms representing the march of progress 

 toward a mechanical perfection in hinge characters, though re- 

 taining traces (as in the striated teeth of some Mactras) of 

 Prionodont ancestry which once dominated the dentition, con- 

 stitute the order Teleodesmacea. 



In the main, in the combination of hinge characters which 

 they represent, the most striking features are the effective 

 manner in which the orthodont laterals and prionodont cardi- 

 nal teeth are subordinated to and supplement each others 

 action, the occasional introduction of the internal cartilage in 

 happy combination with the others and the general absence of 

 nacre in the shell structure and archaic characters in the soft parts. 



It is a question whether the Pudistes are to be considered a 

 group apart, or, like the Pholadacea among the Anomalodes- 

 macea, merely an erratic special development, of forms related 

 to the Chamacea. Leaving the question to be settled by the 

 special studies its difficulties call for, I conclude this paper 

 with a tabular view of the orders and suborders into which the 

 class is divided. One group, the Leptonaoea, stands much in 

 need of thorough study without which its component families 

 and even its permanent standing must remain doubtful. With 

 our present knowledge it is yet impossible to determine the 

 number of families of which each suborder should be com- 

 posed, or even how many groups are entitled to rank as fami- 

 lies. But in the major groups I feel a certain amount of con- 

 fidence that the present arrangement is in most respects more 

 harmonious and in accord with the balance of characters than 

 any of the systematic arrangements of the class which have 

 been hitherto proposed. 



