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TERTIARY FAUNA OF FLORIDA 



really starting-points for numerous subsequently developed genera are usually 

 notable for their tendency to vary and interchange characters. In the present 

 case perhaps the very general habit of commensalism, or parasitism, has pro- 

 duced degeneration or afforded an excessive protection, inducing or accom- 

 panied by a revival of atavistic primary characters. The fact that authors, 

 struck by similarity of dental features to those of immature specimens of genera 

 of widely different origin, have too hastily referred species of Leptonacea to 

 such families as the Mactridw or Cyreuida is significant in this connection. 



It must be confessed at the outset that our knowledge of the anatomy of 

 recent Leptonacea is lamentably deficient. We have to assume (which is never 

 safe) that forms with similar shell characters are generally similar in other 

 points of structure, except where we know to the contrary. We find, moreover, 

 that the dentition is frequently indistinctly developed or somewhat amorphous, 

 rendering it difficult to make out the homologies of the different parts of the 

 hinge. It is certainly unsafe to assume, as Bernard has sometimes done, that 

 the position of a dental lamina is sufficient to settle its homology. The dynamic 

 reactions of teeth upon each other are, I am confident, of the utmost importance 

 in the development of the hinge. As in the vertebrate skeleton, pressure and 

 friction in localized areas will produce directly a response in facets and but- 

 tresses. In fact, to the eye trained to take such matters into account, every 

 hinge shows more or less evidence of the mutability of hinge-structure and 

 its responses to stress, as well as inherited tendencies of form. In no group 

 are these more obvious than in the Leptonacea. 



The prototypic hinge of the group, — or that which with slight modifications 

 will exhibit any of the various types of hinge-structure found in the group, — 

 is very simple and has been figured by Bernard in his illustrations of a minute 

 form which he has named Pachykellya. His invaluable researches upon the 

 early features of the hinge have shown that among the Tcleodesmacea the so- 

 called laterals and cardinals are dissevered parts of an originally single lamina 

 sharply bent at its proximal, or umbonal, end and having somewhat the form 

 of a figure seven (7). In Pachykellia the hinge is composed of an internal 

 resilium not obviously separated from the ligament and inclined obliquely back- 

 ward, as in many nepionic Teleodonts. On each side of this in each valve is 

 a pair of the i:- -shaped lamellae, of which most have developed more or less 

 distinctly the proximal or cardinal " hook." The lower ones are less engaged 

 in the various stresses to which the laminse are subjected in use, and hence, as 

 might be expected, the hook is less evident or even undeveloped. 



From this type of hinge all the others can be developed by trifling modifica- 



