TRANSACTIONS OF WAGNER 



814 



TERTIARY FAUNA OF FLORIDA 



FAMILY PHOLADIDyE. 



This family is in much the same condition as regards the Tertiary 

 American species as the preceding, except that the shells are better known. 

 They are as a rule rare, the specimens more or less imperfect, and often 

 unique, and widely scattered in different collections. Only a few can be 

 considered in detail here, but they will be followed by a list of the species 

 mentioned in the literature. The most thorough revision of the species of 

 the group as regards synonymy is to be found in Tryon's Monograph of 

 the Plioladacea, a reprint of papers extracted from the Proceedings of the 

 Academy of Natural Sciences, Philadelphia, 1861-62, though some shifting 

 of generic names would be necessary to bring the work up to date. 



Subfamily PHOLADIN^E Tryon. 



Anterior hiatus permanently open, with no callum. 



Genus PHOLAS (Linne) Lamarck. 

 Pholas Linn6, Systema Nat., Ed. x., p. 669, 1758 (ex parte) ; Lam., Prodrome, p. 90, 



1799. Type P. dactylus L. 



DactyltnaGr&y, P. Z. S., 1847, p. 187; Tryon, Mon. Pholacl, p. 75, 1862. 

 Phragmopholasfv&vet, Man. de Conchyl., p. 1133, 1887. 

 Thovana (Leach MS.) Gray, P. Z. S., 1847, p. 187. 



Valves with an umbonal reflection, the space beneath it divided into 

 cellular cavities by supporting radial septa. 



Subgenus PHOLAS s. s. 



Shell with an accessoiy protoplax, a mesoplax with the nucleus at the 

 outer margin over each umbo, and a narrow elongate hypoplax. Valves 

 emarginate in front. Type P. dactylus L. 



Subgenus THOVANA Gray, 1847. 



Like Pholas, but with the mesoplax nucleus near the inner margin and 

 the valves regularly rounded in front. Type P. oblongata Say. This is 

 Gitocentrum Tryon, 1862, and based on the same type. 



Subgenus MONOTHYRA Tryon, 1862. 



The shell with a single mesoplax over both umbones with a subcentral 

 nucleus, the anterior hiatus narrow. Type P. oricntalis Gmelin. 



