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TERTIARY FAUNA OF FLORIDA 457 



similar in type in all the species, the differences seen are merely due to the 

 state of development at which they became stationary, and the attitude towards 

 one another of the nepionic laminae. The average dental formula is L.OIO.IOIO.OIO . 

 the anterior left and posterior right cardinals are often bifid, especially in large 

 species like v$\ rivicola. The species are all viviparous. 

 The genus may be divided into the following groups : 



Subgenus Sphcerium Scopoli, s. s.. Type S. corneum Linne. 



The nepionic shell passing into the adult without any distinct demarcation ; 

 the anterior end shorter; the ligament subinternal ; the two right cardinals 

 widely divergent and coalescent at their adjacent or upper ends, thus appar- 

 ently forming but one tooth, but which if it had continued in development 

 would have separated into two; the widening of the ventral angle causes the 

 /\ -shape to disappear; the nepionic shell (and consequently the beaks) is finely, 

 concentrically striate or even nearly smooth and rather convex. 



This group = Sphccriastrum II.*, of Bourguignat, and Cyclas (Corneola}\, 

 of Westerlund. 



Section Cyrenastrum Bourguignat. Type 5\ solidum Normand. 



This section only differs from the preceding by having the laterals cross- 

 striated. It is not more solid and heavy than several other species, and I am 

 puzzled to explain why it has been kept as a separate section by all the authors 

 who have written lately on this group. In examining a large number of speci- 

 mens critically I have found very wide individual variation in most species of 

 Sphcerium in the form and prominence of the cardinal teeth, which may explain 

 why the differential diagnoses of the books are to a marked extent not borne 

 out by the shells themselves.* 



Section Sphceriastrum Bourguignat. Type 5". rivicola Leach. 



Shell resembling Sphcerium s. s. but more coarsely sculptured; nepionic 

 shell polished, concentrically and radially minutely striated ; beaks of the adult 

 with a few shallow, wide, low undulations ; ligament largely exposed ; a simple 

 and a bifid cardinal tooth in each valve, otherwise like Spharium s. s. 



This is the largest form of the group, and in it the development of the car- 



* As, for instance, authentic specimens of S. ovale Fer. are placed by Bourguignat 

 among the species with internal ligament, yet the ligament in most of my specimens is 

 as much uncovered as in C. rivicola. The teeth are said not to be A -shaped and those 

 of 6". corneum are described as A-shaped, yet between some specimens of the two species 

 there is no marked difference, while others have the teeth quite distinct in form. 



