TRANSACTIONS OF WAGNER 

 1 1 to 



TERTIARY FAUNA OF FLORIDA 



tions. The laminae may be long or short ; when the outer limb is short we have 

 a A-shaped tooth ; if the angle proceeds to that stage of development when 

 its continuity is lost, we may have a hinge like that of Cyamiomactra; the 

 severed hook may be modified by pressure to a petaloid shape, which again 

 by degeneration may be reduced to two obscure minute conical projections, as 

 in some species of Galeomma. Any part or the whole of the hinge may become 

 obsolete ; the resilium and ligament may separate or continue in connection ; 

 the latter frequently becomes external and often obsolete, though traces of it 

 almost always exist. 



The arrangement of the groups must, in our present state of knowledge, 

 be provisional. No linear arrangement will show the exact inter-relations of 

 the different genera, and yet we are confined to a linear arrangement. The 

 present tentative scheme is based on our present insufficient information, and, 

 whe*re only shell characters are known, chiefly on those of the hinge. It is 

 difficult at present to say what should be done with Montacuta. According 

 to the literature, it has Lucinoid gills and Thyasiroid hepatic digitations, while 

 the shell is obviously Leptonoid. The anatomical combinations that the other 

 groups would exhibit are at present unknown in many cases. It may be for 

 the present most convenient to place the Montacutas and Aligenas at the end 

 of the list with an unassigned value, as they certainly seem to lead up to the 

 Thyasirida, in spite of the differences of the gills. 



It does not seem practicable to associate Sportella, Anisodonta, and other 

 genera in which the soft parts are permanently retained within the shell, with 

 forms like Galeomma, in which they are exserted, covering a large part of the 

 valves. The only data we have on Anisodonta (quadrata) would indicate that 

 the mantle edges are largely united, the gills as in Thyasira (Cryptodon), but 

 united behind the foot, and, contrary to the rule in the Leptonacea, the incurrent 

 orifice, though not developed into a siphon, is complete and posterior. Yet 

 the shell characters merge so gradually into those of typical Anisodonta, and 

 these into those of Sportella, that one feels that without more definite informa- 

 tion they can hardly be separated. The interchanges of characters, and the 

 multiplicity of forms separated by apparently trifling details of structure, make 

 this group one of the most perplexing I have ever tried to review. 



It should not be forgotten that in certain groups, such as Galeomma and 

 Lascea, individual variation among the teeth is very prevalent within the species. 

 Features which in some other genera might be important are here often of 

 no systematic importance whatever, and are liable to lead the " closet natu- 

 ralist" into serious error. 



