INSTITUTE OF SCIENCE, PHILADELPHIA. 95 



same width as the interspaces (the anterior ones the broadest), with an 

 interstitial secondary rounded rib in the centre of the interspace ; the 

 two valves unequal, the basal margin of the left valve greatly protruding 

 beyond that of the right; base profoundly crenulated. 



Length, 3.3 inches ; height, 2.5 inches. 



Abundant in the banks of the Caloosahatchie below Fort Thompson. 



I am not absolutely satisfied as to the value of this species, although 

 the form, so far as all the specimens collected by us are concerned, is a 

 very clearly defined one. It closely resembles the shell identified by 

 Tuomey and Holmes with Area scalaris of Conrad (Foss. Med. Tert. Form. 

 U. S., p. 59, pi. 3 1 , fig. i ; Tuomey and Holmes, Pliocene Fossils of South 

 Carolina, p. 43, pi. xvi, figs. I, 2), and might, indeed, be readily mistaken 

 for it. Through the kindness of Prof. Whitfield I have been permitted to 

 make a comparison with the type-forms described and figured by Tuomey 

 and Holmes, and find that their shell differs very materially from the 

 Florida fossil. In the first place it is decidedly more oblique, and 

 secondly, the ribs adjoining the posterior slope (on the left valve) are not 

 nearly as broad relatively, nor as flattened, as they are in A. scalarina ; the 

 ribs of the left valve are more remotely placed from one another, and lack 

 the pronounced interstitial secondary rib, which is so prominently defined 

 in the Florida fossil. Its place is taken by a hair line, which is present 

 in same of the intercostal spaces. The characters of the Florida shell 

 are remarkably constant, showing practically no variation, and were I as 

 positive of the stability of characters in the Carolina fossil, I should have 

 no hesitation in regarding the two as specifically distinct; unfortunately, 

 only a single pair of valves of Tuomey and Holmes's shell has been posi- 

 tively identified, which, therefore, gives no information on this point. As 

 it is, the characters of the two are sufficiently distinct, indeed, fully as 

 well-marked as those which separate the Florida fossil from the recent 

 Area incongrua of the Southern coast, which may, with much plausibility, 

 be looked upon as its immediate descendant. The recent species agrees 

 more nearly in the general outline of the shell, being upright rather than 

 oblique, but differs in the less width (in the left valve) of the ribs, and in 

 lacking the true interstitial rib of the right valve (although an indication 

 of it appears in a faint elevated line), agreeing in this respect with the 

 South Carolina fossil. That the three forms are most intimately related 

 there can be no question, and I believe there is likewise little or no 

 question that they all lie on the same line of descent. Tuomey and 

 Holmes assert that their shell is closely allied to Area transversa. This 

 is certainly a mistake; the two shells, beyond the general characters 

 uniting the majority of Areas, have very little in common neither in 

 shape, size, nor ornamentation. 



Another point that remains to be determined in this connection is 



