TRANSACTIONS OF WAGNER 

 1338 



TERTIARY FAUNA OF FLORIDA 



veloped on the hinge apart from the lunular nodosity, though as in other edentu- 

 lous bivalves it frequently happens that minute irregular nodulations may occur 

 at the inner (proximal) end of the ligament below the beaks. 



Some of the forms of Ax'mopsis, as already stated, arrive at a point where 

 they may be said to have true cardinal teeth, though these appear to originate 

 by the infolding of the anterior sublunular hinge-margin and not by the de- 

 velopment from primordial lamellae, as in most Pelecypods. In Leptaxinus 

 we have also well-developed laterals, and in one of the species, L. incrassatus, 

 Jeffreys has figured what appears to be a provinculum. 



The extraordinary arborescent hepatic and genital glands appear to be 

 characteristic of this family, and have been shown by Friele (1877) to exist 

 in A.rinopsis, a fact which I have confirmed. 



I have noticed in the young a much greater variation in outline than occurs 

 in the adult, and it therefore becomes necessary in dealing with very minute 

 forms to assure oneself that one is dealing with mature animals before assign- 

 ing too high a systematic value to such mutations. 



The family may conveniently be divided into the following genera and 

 minor groups : 



Genus Thyasira (Leach). Type T. flexuosa Montagu. 

 Section Thyasira s. s. 



Valves with edentulous hinge, the anterior dorsal area more or less im- 

 pressed, the posterior more or less distinctly plicate or radially sulcate. 



Philis (Fischer), 1861, has a deeply impressed lunule, but this character 

 intergrades with the other species. 



Megaxinus Brugnone, 1881, differs only in having the ligament strong and 

 deeply inset (not occluded), much as in Codakia, and supported by a more 

 prominent nymph than in the ordinary Thyasira. 



Section Axinulus Verrill and Bush, 1898. Type T. (A.} brevis V. and 

 B., Proc. U. S Nat. Mus., xx., p. 790, pi. Ixxxix., figs. 7, 8, 1898. 



Shell minute, ovate or oblong, with the dorsal areas obsolete. 

 Axinodon Verrill and Bush, 1898, appears to be synonymous. 



Genus Axinopsis G. O. Sars, 1878. Type A. orbiculatus Sars, Moll. Reg. Arct. 

 Norv., p. 63, pi. xix., figs, n a-d, 1878. 



Valves with one or more cardinal teeth, shell small, solid, with no posterior 

 dorsal area or plication. 



