FREE INSTITUTE OF SCIENCE 



TERTIARY FAUNA OF FLORIDA 



lamellation ; the shell is frequently subtruncate behind or more or less quad- 

 rate; internally the margin is always white, but sometimes in the cavity of 

 the beaks a pretty, pale-purple suffusion is seen, which no specimen of V . mer- 

 cenaria is known to exhibit. The brown zigzags are distinctly linear, while in 

 V '. mercenaria they are broader and more or less blotchy. . 



The mutations are much the same as in V. mercenaria except that the middle 

 of the disk is never wholly smooth, the lamellae are usually continuous, and the 

 change, if any, in the middle of the shell consists in broadening the lamellae 

 themselves, on the polished tops of which the brown painting will appear with 

 great distinctness. In full-grown specimens the lamellae are always pretty 

 close set, but in the young they may be fine and close, or coarse and distant. 

 The posterior radial sulcus in senile specimens is strong. Occasionally an 

 adult shows traces of purple coloration at the margin, which may be due to 

 hybridity, but I have never seen any specimens with the other characters of 

 V. campechiensis which had the dark purple border of the typical V. mer- 

 cenaria. 



The measurements of a fully adult specimen are as follows: length 133, 

 width 117, diameter 82 mm. 



The species is not positively known to be now represented north of Chesa- 

 peake Bay ; it is the prevalent type beyond the Mississippi delta on the Texas 

 coast, and the only one yet reported from Yucatan, where Schott collected it 

 abundantly. It was from this region that Lister's adolescent shell, which 

 served as Gmelin's type, was originally obtained. Varieties may be distin- 

 guished as follows: I, variety alboradiata Sowerby, with brownish rays on a 

 paler ground ; 2, variety quadrata Dall, with the shell thin, small, compressed, 

 subquadrate, and unicolorate ; 3, variety texana Dall, with the concentric 

 lamellae towards the middle of the disk coalescent, forming broad, more or 

 less inosculating, flat-topped ribs with polished tops ; the valves usually very 

 convex. Numerous names have been given by Conrad to the mutations of 

 the fossil form, many of which mutations can be found in a large collection of 

 the recent shells. Of these V . tetrica Conrad is large, subtrigonal, rather mod- 

 erately convex, and with less prominent beaks than the type, but it has the 

 close, fine lamellation of the typical V . Mortoni, which is the fully adult V. 

 campechiensis. V. permagna Conrad is ovate instead of trigonal and rather 

 longer and less inflated than the type, with some of the lamellae thickened 

 medially ; V . capa.v Conrad is the suborbicular and ventricose young of V. 

 Mortoni which have passed the subquadrate stage. The name of V. submortoni 

 was substituted for V. Mortoni Conrad by Orbigny because the latter included 



