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IO71 



TERTIARY FAUNA OF FLORIDA 



the interspaces of the ribs externally) internally radiating from near the 

 umbones, where at their terminations they are, as it were, roofed over ; these 

 channels are differently colored from the rest of the interior. Somewhat 

 analogous is the case of Ethmocardium, an Upper Cretaceous type, in which 

 the region within the pallial line is deeply pitted, the apices of the conical 

 pits being covered with an extremely thin layer of shelly matter, so that the 

 least exfoliation or erosion results in the appearance of a row of perforations 

 in the channels between the external ribs over a large part of the disk. In 

 most Cardiacea the pallial line is more distant from the distal margin of the 

 valves than is usual in Pelecypoda, and it is frequently subtruncate behind. 

 In Leptocardia, a small Cretaceous type, there is a double sinuation of the 

 posterior part of the pallial line, almost suggesting a pallial sinus, and in 

 Serripes it is truncate. 



The periostracum in most species is thin and obscure, but in the boreal 

 species, except Serripes, it becomes more conspicuous, coarse, and even tufted. 

 A few tropical forms, notably C. latuin Reeve, also show a pubescent peri- 

 ostracum. 



The ligament, which encloses an obscure resilium, is usually short, strong, 

 and seated in a deep groove, forming short, often thickened, nymphs. I have 

 not found any species with an amphidetic ligament or any tendency to a sink- 

 ing in of the ligament. The activity of these animals is such that the valves 

 must be pretty flexible in their motions not to put their owner to a disadvan- 

 tage among its kind, and this condition is correlated with the feebleness of, 

 the cardinals, the strong, short ligament, and the constant presence of serra- 

 tions to hold the valves in place when closed. 



Nearly all Cardia have two forms, one more equilateral and globose, the 

 other more oblique and elongated, but whether these differences can be corre- 

 lated with sex is at present unknown. 



The shell of Cardium, especially the tropical species, is frequently furnished 

 with an external shelly layer from which most of the spinose, nodose, or 

 other superficial sculpture is wholly formed. This layer is very feebly attached 

 to the next inward layer of shell and may be easily scaled off, taking with it 

 the sculpture. The surface below it is usually polished, and in fossils, espe- 

 cially of the subgenus Fraguin, the ribs will then appear polished and perhaps 

 flat, when they were originally keeled or nodose, and show no evidences of 

 erosion. This deceptive habit should be borne in mind by workers on this 

 group. 



The distribution of the genus in time among our Tertiaries has some 



