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39°'] PROCEEDINGS OF THK NATIONAL MUSEUM. 245 



questioned. Through them we pass to the Pectinacea, in which iu 

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

 teeth, as Area is of one with many teeth. 



The original transverse 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 then- characters, though so far modified as to indicate 

 little, by the hinge, of what I suppose to be their origin. Above all it 

 must be admitted that the Monomyaria and Heteromyaria represent not 

 fundamental types of structure but special modifications, though some 

 of them are geologically ancient. 



The remaining forms representing the march of progress toward a 

 mechanical perfection in hinge characters, though retaining traces (as 

 in the striated teeth of some Mactras) of Prionodont ancestry which 

 once dominated the dentition, constitute the order Tdeoilesmacea. 



In the main, in the combination of hiuge characters which they pre- 

 sent, the most striking features are the effective manner in which the 

 orthodont laterals and prionodont cardinal teeth are subordinated to 

 and supplement each other's action, the occasional introduction of the 

 internal cartilage in happy combination with the others, and the gen- 

 eral absence of a prismatic layer and of nacre in the shell-structure and 

 of archaic characters iu the soft parts. 



It is a question whether the Eudistes are to be considered a group 

 apart, or, like the Pholadacea among the Anomalodesmacea, 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 Leptonacea, stands 

 much in need of thorough study, without which its componeut families 

 and even its permaneut standing must remain doubtful. With our 

 present knowledge it is yet impossible to determine the number of fam- 

 ilies of which each suborder should be composed, or even how many 

 groups are entitled to rank as families. But iu the major groups I feel 

 a certain amount of confidence 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. 



Supplementary note. — When I first began to consider the rela- 

 tions of the teeth and other parts of the hinge I naturally remembered 

 the brief abstract of the important paper on the hinge of bivalves by 

 M. Neil may r, which I had seen in the Zoological Kecord for 1883. I 

 intentionally deferred a careful perusal of Neuinayr's essay until I had 

 entirely completed my own. Then a careful examination of his original 

 afforded me great pleasure. It showed that in the matter of the influ- 

 ence of ribbing in promoting nasceuce of teeth; in the discrimination 



