Dall — Hinge of Pelecypods and its Development. 453 



orthodont influence, I have used the term Prionodesmacea. 

 In some cases what may seem to be the chief features of the 

 hinge as regards size and strength are orthodont, yet these I 

 believe to be comparatively modern specializations illustrating 

 the general tendency of evolutionary processes toward a teleo- 

 dont hinge. In cases of doubt the sum of the characters will 

 enable us to decide on the proper place, for a given genus. It 

 must not be supposed that, because the names suggested by a 

 single set of characters are used to denominate the proposed 

 orders, that therefore that set of characters is to be our sole cri- 

 terion. Such too hasty assumptions are a relic of the days 

 when the immutability of species was an orthodox dogma in 

 biology, and doom to failure any system founded upon them. 



For those forms in which the various types of hinge have 

 become harmoniously combined, though in varying proportion 

 contributing to the final mechanism, I have selected the desig- 

 nation of Teleodesmacea. These may be regarded as the 

 highest and evolutionaily the most perfect in type of hinge, 

 though this perfection shows itself in a variety of forms. 

 Prionodont traces remain with most of them but are never 

 characteristic of the type. 



The three groups 1 propose to call Orders. It is difficult to 

 say whether' they can be compared in systematic value with 

 orders in other classes. All that can be said is that these three 

 divisions are discernible in the very compact and homogeneous 

 class which includes them, and it contains no other groups of 

 equal value or significance. 



Each Order as it now exists contains archaic and modern spe- 

 cialized types. Each indicates a tendency toward an ideal of 

 fitness to the environment, which results in a certain parallelism 

 of minor characters common to minor groups in each of the 

 three orders. In each (we are coming to regard it as inevitable), 

 certain members show affiliations with members of the other 

 orders. In each there are certain groups which represent a 

 relatively modern specialization carried so far as to be quite 

 peculiar. 



Pearliness or a truly nacreous character of shell substance is 

 a source of weakness. This kind of shell is more fully per- 

 meated with animal matter, is more liable to decay and exfolia- 

 tion and is more readily drilled by enemies than the arragonitic 

 type of shell substance which conchologists call porcellanous. 

 The tendency of evolution is to promote the porcellanous 

 type. The older groups {Prionodesmacea and Anomalodes- 

 maeea) contain all the pearly Pelecypods, among the Teleodes- 

 macea there is not a single one. Furthermore, in the two 

 former orders the most specialized and, developmentally, the 

 most modern forms are preferably porcellanous; those which 



An. Jodk. Sci.t-Third Series, Vol. XXXVIII, No. 228.— Dec, 1889. 

 29 



