TRANSACTIONS OF WAGNER 

 1058 



TERTIARY FAUNA OF FLORIDA 



burrow in sand have the shell elongated, and this elongation, varying in 

 amount in different species, is accompanied, as usual in such cases, by an antero- 

 posterior protraction of the soft parts. For these species Stoliczka has pro- 

 posed the name Petricolaria. Lastly we have boring species, in which the 

 natural inequality of the valves is exaggerated and the margin of the right 

 valve in full-grown specimens is irregularly expanded, overlapping that of 

 the left valve, which remains normal, and frequently forming channels in 

 which the siphons lie or may be extruded. For these forms, erroneously re- 

 ferred to Venerupis, Fischer has proposed the subgeneric name of Claudi- 

 concha. These also sometimes have broken teeth cemented. Petricola in the 

 wide sense and adult state has no lateral teeth ; in the left valve there are 

 three radial cardinals, the middle one larger, higher, and bifid, or with several 

 grooves ; in the right valve there are two cardinals, the anterior simple, 

 arcuate, and often very prominent, the posterior lower, oblique, and grooved 

 or bifid. The resilium and ligament are coincident and external on nymphs ; 

 the lunule is absent or ill-defined ; the pallial sinus small or large, the siphons 

 separate, elongated, and naked. 



From the often very similar Venerupis the species of this group may be 

 separated by the hinge-teeth and generally by the absence of regular and 

 elevated concentric lamellae. Venerupis has three subequal, usually bifid teeth 

 in each valve, and the margin of the shell is very commonly serrate or 

 denticulate, which is not the case in Petricola. It should be borne in mind 

 that in these more or less distorted borers or nestlers the hinge in fully 

 grown shells is almost always more or less distorted and defective ; only the 

 examination of a large series, especially of the young shells, can give an 

 adequate idea of the normal dentition. 



The genus appears to be divisible into sections as follows : 



Section Petricola Lamarck, s. s. Type P. lapicida (Gmelin). 

 Shell ovate, with a short or moderate wide pallial sinus, the radial sculp- 

 ture more or less divaricate or zigzag. Naranio is synonymous. 



Section Rupellaria Fleuriau. Type P. lithophaga (Retzius). 

 Shell inflated and rounded in front, attenuated and more compressed be- 

 hind ; sculpture chiefly radial, stronger anteriorly. Lajonkairia and Choris- 

 todon are synonymous. 



Section Claudiconcha Fischer. Type P. monstrosa (Gmelin). 

 Margin of the right valve irregularly expanded, pallial sinus shallow, form 

 like Petricola. 



