V °1889. n '] PROCEEDINGS OF THE NATIONAL MUSEUM. 239 



Attention has already been called to the fact that there can be but 

 three fundamental types of binge, which may be called the auodout, 

 prionodont, aud orthodont, the latter term beiug used to indicate the 

 forms in which the cardinal margin has become longitudinally plicate. 

 Actually the pure orthodont type hardly exists; in nearly all forms 

 traces of the prionodont characters are mingled with it. For those 

 forms, in which the archaic anodontism still persists as the characteristic 

 of chief importance, though frequently modified by special mechanical 

 contrivances which to a certain extent mask the type, I have proposed 

 the term Anomalodesmacea. The fossette, cuilleron, or spoon-shaped 

 process for the cartilage is a separate development, serving a special 

 purpose. Though influencing the teeth, if any exist, in its vicinity, it 

 must not be confounded with them. The weakness of the auodout 

 type has left an opening for the specialization and perfection of this 

 process, which, to a considerable extent, iu this group, assumes the 

 functions which in groups without a cartilage are the special office of 

 the teeth. 



For those forms in which transverse plication of the hinge is the 

 chief characteristic, though rarely wholly exclusive of the orthodont 

 influence, I have used the term Prionodesmacea. Iu 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 proc- 

 esses toward a teleodont hinge. Iu cases of doubt the sum of the 

 characters will enable us to decide on a 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, therefore 

 that set of characters is to be our sole criterion. Such too hasty assump- 

 tions 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 iu varying proportion contributing to 

 the final mechanism, I have selected the designation of Teleodesmacea. 

 These may be regarded as the highest and evolutionally the most per- 

 fect 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 I 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 discern- 

 ible in the very compact and homogeneous class which includes them, 

 aud it contains no other groups of equal value or significance. 



Each order as it now exists contains archaic and modern specialized 

 types. Each indicates a tendency toward an ideal of fitness to the en- 

 vironment, which results iu a certain parallelism of minor characters 



