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1313 



TERTIARY FAUNA OF FLORIDA 



places to Cape Cod, southward to the Florida Keys, and westward to the Texas 

 coast, where it is rare. The variety notata has been collected from New Eng- 

 land to Georgia. It has been introduced into British seas. 



The remarks and synonymy connected with this species by Linne, Pen- 

 nant, Dillwyn, and other early writers indicate some confusion of it with Venus 

 (^ Cycles) islandica, and a consequent extension of its distribution to north- 

 ern Europe. Later this was cleared up. In studying the recent specimens to 

 obtain light on their fossil predecessors, I came to the conclusion that two 

 species can be distinguished, with different ranges of distribution, and which 

 have an analogous series of mutations and very possibly, in the region where 

 both occur, occasionally hybridize. 



V. mercenaria in its typical form is more produced and pointed before and 

 especially behind ; it has the concentric sculpture obsolete on the middle of the 

 disk, where the shell is often tinted with ferruginous brown and, if living in 

 mud, with bluish black. The lunule and escutcheon are not differently colored 

 from the rest of the shell. The pallial sinus was regarded by Stimpson as 

 shallower than in V. cmnpechiensis, and its ventral boundary usually makes a 

 more obtuse angle with the pallial line, though I do not find the differences in 

 this respect very marked. The purple coloring when present is marginal, and 

 some trace of it almost always persists on the dorsal posterior margin. In the 

 young over an inch long the shell is rounder and more convex than in campe- 

 chiensis of the same age, while in the adults these characters are reversed. In 

 the very large series of specimens which I have studied the crenulation of the 

 margin in V. mercenaria seems to me to be quite constantly coarser than in 

 V. campechiensis of the same size. 



The shell grows rather slowly. In specimens " planted" in a favorable 

 locality in September, 1879, the median radius was 31.0 mm. ; in the following 

 April 32.5 mm. ; in June, 1880, 35.0, and in September, 1880, 45.0 mm. This 

 shows a growth in one year of 14.0 mm., most of which took place between 

 June and September. Judging by the incremental irregularities, a specimen 

 with a median radius of 11 5.0 mm. from the summit of the beaks to the middle 

 of the base (measured in a straight line on the inside of the shell) was about 

 ten years old. 



The dimensions of a fully grown normal specimen are as follows : length 

 133, height 104, diameter 50 mm. 



The observed mutations are as follows : 



1. Closer or more distant concentric lamellation. 



2. Rounder or more cuneate form. 



