INSTITUTE OF SCIENCE, PHILADELPHIA. 281 



Cerithium platynema n. s. 

 Plate 14, figure 11. 



Uppermost Eocene (or lowest Miocene) shell limestone at Jacksonboro', 

 Scriven Co., Georgia, Prof. W. B. Clark. 



Shell represented in a limestone chiefly composed of fragments of small 

 Pelecypoda, by molds from which gutta-percha casts were made and the de- 

 scription and figure taken from these. 



Shell large, nucleus lost, with eight or more subsequent, rounded whorls ; 

 transverse sculpture of about fifteen narrow, little-elevated, rounded ribs, which 

 become less evident toward the sutures and are separated by equal or wider 

 interspaces ; these are crossed by fine, even, close-set spiral secondary threads, 

 by three stronger primary, flattened threads near the middle of the whorl, and 

 by a fourth strong, plain primary thread on which the suture is wound ; toward 

 the end of the last whorl the primaries are somewhat more elevated and the 

 secondaries less prominent; the intersections of the ribs and primary spirals 

 are prominent, but hardly nodulous ; on the last whorl a new, flattened primary 

 appears just in front of the suture; the varices are perceptible, but not promi- 

 nent on the spire. Lon. of specimen figured 26; max. diameter 11 mm. 

 Another stouter specimen measures 30 by 15 mm. 



The sculpture of this species is so well marked and peculiar that it can- 

 not be mistaken for any of the others, even without a knowledge of the char- 

 acteristics of the aperture and base. 



The Ceritliiuin georgianiiin Lyell and Sowerby was described from this 

 locality, and at first I thought this species might be identical with it, but on 

 further study this seemed unlikely, as the sculpture of the two forms differs. 

 The upper layer in which C. platynema was found seemed to differ from the 

 other rocks by the absence of characteristic Eocene types, and the suspicion 

 was awakened in the minds of both Prof Clark and myself that this upper 

 layer might be part of the earliest Miocene deposited upon the older Eocene 

 formation, though the fauna was not sufficiently abundant or characteristic 

 to decide. Lyell referred the whole to the Eocene. 



Cerithium sp. indet. 



With the preceding, at Jacksonboro', Scriven Co., Georgia. 



Another species with well-defined sculpture is indicated by gutta-percha 

 casts from the same block as that which contained C. platynema. It is charac- 

 terized by about ten narrow, sharply defined, slanting ribs, rounded, with about 

 equal interspaces and extending across the flattened surface of the whorl from 

 suture to suture. Over these are carried three flattened, broad, little-elevated, 

 strap-like spirals with narrower, channelled interspaces. There are no nodula- 

 tions at the intersections. Behind the suture is a smaller spiral on which the 

 whorl is laid. It is probable that there may be some fine spiral striation on 

 the shell, but the cast shows hardly any indication of it. The cast repre- 



