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651 



TERTIARY FAUNA OF FLORIDA 



Area Holmesii Stimpson, S. I. Checklist, p. 2, i860 ; Tryon, Am. Mar. Conch., p. 179, 



pi. 37, fig. 471, 1874. 

 Not Area aincricana Orb., Moll. Cuba, ii., p. 317, pi. 28, figs, r, 2, 1853. 



This very interesting species, of which the synonymy might be much 

 extended, affords an excellent illustration of the effects of environment upon 

 the recent form. Its northern Hmit is at Cape Cod, where the shell is often 

 large, always coarse, and with a dense hirsute periostracum. Like many of 

 the Scapharcas, it varies in outline from quite round to ovate quadrate ; the 

 sculpture of the two valves is discrepant, that of the left valve showing ribs 

 which are narrower, flatter, and less prominent than those of the other valve 

 and often impressed in the middle longitudinally, or even divided by a mesial 

 groove more or less extended from the margin. The ribs of the other side 

 are not grooved, and the literature is so at variance with itself and the facts, 

 in attempts to discriminate the several varieties, that I can only suggest as an 

 explanation that the writers in some cases were unaware of the discrepancy 

 between the valves and compared opposite sides. As we proceed southward, 

 in this species, as in many other shells, we find the shell becoming less earthy 

 and more porcellanous, the sculpture more neat, the periostracum less pro- 

 fuse, and the general size less. South of Cape Hatteras the chalky, thin type, 

 common in the north, is seldom if ever found. In the Gulf of Mexico and 

 the Antilles the shell is still smaller than in the Carolinas, and, with its de- 

 crease in size, the sulcation of the ribs becomes more generally obsolete. A 

 somewhat similar series of differences is observable in the Pleistocene fossils, 

 though less pronounced. 



Gmelin's description was inadequate, and only identifiable by his reference 

 to Lister. The species was elucidated by Dillwyn, who noted its resemblance 

 to Cardium (j:dule), but whose reference to a figure in the Encyclopedic Meth- 

 odique should be expunged from the synonymy. 



The typical A. campechensis is the rounded southern form which Stimpson 

 afterwards called A. Holmesii, as he himself recognized. Say's description of 

 A. pcxata included all the varieties of our eastern coast, but Gould first de- 

 scribed the shell so as to make this name apply more particularly to the 

 somewhat elongated, earthy northern variety. Gray's A. americana was 

 founded on a very elongated, more porcellanous form, such as is common in 

 South Carolina waters. The study of a large series of recent specimens, 

 ranging from Jamaica to Cape Cod, obliges ine to recognize that no sharp 

 line of discrimination can be drawn between the several varieties. The number 



