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721 



TERTIARY FAUNA OK FLORIDA 



C. Variety edgecombtnsis. Seven with twelve, one with thirteen, five with 

 fourteen, two with fifteen, two with sixteen, one with seventeen, and 

 one with twenty ribs. 



The typical number for the species, therefore, would seem to be nine or ten 

 primary ribs. 



The surface is covered with fine radial threads, and in this species they 

 arc very close together, even in size, closely set with small raised scales. 

 The fact that these scales are so close to one another makes the transverse 

 lines pretty even. In Madisonius the threads are larger, the scales larger and 

 more sparsely distributed on the threads, so that they frequently have an 

 alternated aspect. 



The threads are usually very uniform in size in Jcffcrsonins, but it fre- 

 quently happens in the specimens with more numerous primary ribs that the 

 middle threads in the interspaces will be somewhat larger than the others. 

 This is not very apparent in the specimens which retain the scales perfect, 

 but in those which are worn the interspaces seem to have a distinct mesial 

 thread. It was to this kind of mutation that Conrad gave the name of 

 edgecombensis, the type-specimens of which are in the National Museum. 

 When there are fewer primary ribs, as in the type of the species or the variety 

 septenarius, the threads are more uniform over the wider ribs and inter- 

 spaces. 



The most conspicuous character by which the peripheral specimens of 

 Jeffcrsoniits can be discriminated from those of Madisonius is comprised in the 

 sculpture and form of the byssal ear. In Jeffersonius it is sculptured with fine, 

 uniform, numerous threads, and the notch is shallow and leaves an incon- 

 spicuous fasciole. In Madisonius the upper part of the ear is provided with 

 comparatively few and coarse threads, and the notch is wide and deep, with 

 a broad and well-marked fasciole. Counting the ribs of seventy adult speci- 

 mens df Madisonins in the collection, the following result was obtained: 



A. Variety Madisonius s. s. Four with twelve, five with thirteen, eight with 



fourteen, twenty-three with fifteen, fifteen with sixteen, nine with seven- 

 teen, and two with eighteen ribs. 



B. Variety Sayanns. One with thirteen and four with fifteen ribs. 



It would seem, therefore, that the normal number of ribs in Madisonins is 

 fourteen to seventeen. 



The two species usually, but not invariably, differ in convexity, Jtffcr- 



ii 



