FREE INSTITUTE OF SCIENCE 



871 



TERTIARY FAUNA OF FLORIDA 



ology of the hinge of bivalves.* Having independently arrived at conclu- 

 sions in regard to many points in this connection agreeing with those deduced 

 by Professor Neumayr, it will not be due to any want of appreciation of his 

 talents that I find myself in some particulars unable to follow him as regards 

 details. In accepting most of his views in regard to the initiation of the 

 various types of dental armature, — and, among others, that which derives 

 some part of his so-called Desmodont type of tooth from plications correlated 

 with the submergence of the protoligament, a view independently proposed 

 by me in 1889, — I cannot accept his homologies of the paits of the Mactroid 

 hinge considered in detail, nor do I believe that his Desmodonta form a valid 

 group. In fact, a fuller consideration of the types of hinge displayed by 

 recent Pelecypods has shown the distinguished and lamented Austrian the 

 fallacy of this earlier conclusion. f The secondary submergence of the liga- 

 ment is a dynamic process which might be looked for and does occur in all 

 the groups of bivalves to which I have attached an ordinal significance, in- 

 cluding the Neumayrian Desmodonta, Taxodonta, Heterodonta, and Dysodonta ; 

 and the association with this submergence of plications ,which give rise to 

 teeth is a dynamic result which depends solely upon the efficiency of the 

 hinge through which the submergence takes place. J When, as in the case of 

 the Prionodonta (Taxodonta), the rigidity of the valves is sufficiently provided 

 for by a long series of interlocking processes, there is no demand for the 

 development of additional guards to the desired stability of motion in a trans- 

 verse vertical plane, and hence their development in such forms as the taxo- 

 donts is not to be expected. But where the hinge is imperfectly provided 

 with dental leaders, and their place is not supplied by the presence of an 



* Sitzb. Kais. Acad. W!ss: Wien., Ixxxviii., p. 3S5, 1883. 



f Cf. Morph. der Biv. Schal. 



J Bernard has shown, in his interesting and important studies of the development of the hinge 

 of bivalves, that the nepionic ligament lies between the thin edges of the valves and more or less 

 obliquely across them, so that it is both internal and partly external. The changes by which the 

 ligament and resilium become wholly external in many bivalves must involve an elevation of the 

 organs mentioned. The paleontological history of such forms as Crassatellites shows that their pre- 

 cursors had an external, or nearly external, ligament; and, as we follow the. members of the group 

 in time, the ligament and resilium become larger and more internal in the successive species, until the 

 modern type is reached. Therefore the internal position in such cases is properly referred to as the 

 result of submergence, notwithstanding the fact that it is partly a return to a state originally normal 

 and universal. Crassatellites, Mactra, etc., are not forms in which the originally sunken ligament 

 has specially developed in place, but in which it has risen to the exterior and been again submerged. 



