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999 



TERTIARY FAUNA OF FLORIDA ^^^ 



lariaj which it seems to represent on the American coasts. It* differs from the 

 type of Scrohicularia, which hves in sandy places, by being found as a nestler ; 

 though never excavating burrows in hard substances, it often occupies those 

 made by true borers, and in this way exhibits a great diversity of outline 

 within the species, as usual with nestlers. The surface is usually fine, radially 

 striate or sagrinate, with concentric sculpture which may be in one and the 

 same species mere lineation or elevated lamellse, the different mutations of 

 sculpture frequently occurring, at different stages of growth, on the same 

 specimen. The right valve exhibits two strong lateral teeth, the anterior one 

 distally being often subspinose, the dorsal margins of the left valve are ex- 

 tended to fit in the channels above the laterals of the opposite valve, the 

 outer surface of these extensions forming a lunule and escutcheon almost 

 wholly confined to the left valve. There is an external ligament and strong 

 internal, posteriorly directed resilium. The pallial sinus is deep and well 

 marked, the siphons separate and naked, the gills as in Scrohicularia. The 

 genus has its emporium on the two coasts of middle America and extends 

 in the Pacific to Simoda, Japan. A subgenus, Thyella H. Adams, 1865 (not 

 R. Desvoidy, 1863), is represented by its type, T. elegans Sby., in the PhiHp- 

 pines, and a fine species, T. Stimpsoni Dall, in the Loochoo Islands. It differs 

 from Cumingia in the absence of lateral teeth in the right valve. The genus 

 Moiitrousieria Souverbie has somewhat analogous hinge characters, but is not 

 a nestler and may not be closely related to Cumingia. It is represented by 

 a single species in New Caledonia. The number of species of Cumingia has 

 been overestimated, owing to the variability of its characters due to the nestling 

 habit. In the northern range of ~ the common species of the United States on 

 both the Pacific and Atlantic we find the shells larger and the sculpture less 

 sparse and irregular. As we follow the species south the shells seem to 

 diminish in average size and the lamellation becomes relatively more promi- 

 nent. Thus, south of Florida the specimens never attain the size of those 

 of the Carolinas and Massachusetts, and on the Pacific the northern speci- 

 mens of C. californica are twice as large as those of the Gulf of California 

 and Panama. Though the change is gradual, and I am inclined to believe 

 all the mutations should be referred to one species, I have kept them separate 

 here for convenience, as the extremes differ considerably. 



Cumingia medialis Conrad. 

 Cumingia tellinoides Conrad, Fos. Med. Tert., p. 28, pi. i.S, fig. 4, 1838; not of Conrad, 



