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lOQI 



TERTIARY FAUNA OF FLORIDA ^ 



these ribs overhang so much as to nearly roof over the narrow channelled 

 interspaces; on the upper fifth of the shell the sides of the ribs, and on 

 the umbo their tops, are gently crenulated. Internally the margins in front 

 and below are fluted, tlte sulci extending half-way up the shell ; the posterior 

 margin is distantly denticulated by the ends of the ribs of the posterior area. 



A singular feature, and one which seems to be worthy of sectional dis- 

 crimination, is presented by this species, though not mentioned in the original 

 description. Internally, from the umbo, nearly in the middle of the shell a 

 stout, elevated, solid rib is given off and extends downward on the shell wall 

 to about the level of the lower end of the posterior lateral tooth. An exam- 

 ination of C. elongatum and many other exotic species of Cardium revealed no 

 such feature in any, though C. pseudolima and a few other species have the 

 thickened ridge in the wake of the posterior adductor scar common to so many 

 bivalves. 



While this rib is conspicuous only in full-grown shells, it exists in the 

 youngest hitherto examined. 



The measurements of the largest pair of C. Dalli in the National Collection 

 are: Alt. 136.0, Ion. 93.0, diam. 51.0 mm. 



In concluding this review of the Trachycardia it may be mentioned that 

 the enormous C. quadragenarium Conrad, 1838, is reported by Cooper to occur 

 in the Pliocene and Pleistocene beds of California as well as in the existing 

 fauna from San Pedro southward. It is the C. luteolabrum of Gould, 1851, 

 and C. xanthocheilmii (Old. MS.) Carpenter, 1856. 



Subgenus RINGICARDIUM Fischer. 



Cardium procerum Sowerby (1833, of which C. laticostatum Sowerby is 

 said to be the young, and C. panamense Sowerby, 1843, very closely allied) is 

 known from the Pliocene of the well in the City Park at San Diego, California', 

 and also from the Pleistocene of the coast. In the recent fauna it ranges from 

 Lower California and the Gulf to Panama. 



Subgenus CERASTODERMA Morch. 



This group contains the greater number of our Neocene cockles and is 

 by far the most imposing in point of size. Of its two sections, one, the typical 

 group, is circumboreal in distribution ; the other, Dinocardium, is, so far as 

 I know, exclusively American and confined to the warmer waters of the 



