268 UROCOPTIS, S. G. GONGYLOSTOMA. 



fig. 5, U. scalarina) is chiefly notable for the great reduction 

 in the number of teeth in a transverse row, and the degenera- 

 tion of all but the inner two lateral teeth on each side. The 

 radula is in process of specialization exactly parallel to that 

 of Brachypodella, toward a form in which the inner four 

 lateral teeth alone are functional; but the result cannot be 

 exactly the same because in Tetrentodon this specialization 

 is superposed upon a phylum having teeth already diversely 

 specialized from the ancestral stock whence Brachypodella 

 and Urocoptis arose. It should be noted also that this group, 

 parallel to Brachypodella in both shell and dentition, arose 

 in an area where Brachypodella does not exist. 



The shape and structure of the cusps of the individual 

 teeth are exactly as in Cochlodinella and Gongylostoma ; but 

 in some species the posterior angles of the basal-plates (lower 

 angles, as figured) are strongly thickened. This is well 

 shown in fig. 5, representing a central and an inner lateral I 

 tooth of U. scalarina in profile, the two posterior-lateral pro- 

 cesses of the central tooth projecting at the lower left side 

 of the figure, while the thickened outer angle of the lateral j 

 tooth projects like a cusp, below on the right. I have exam- 

 ined the teeth of U. cyclostoma, sexdecimalis, camoensis, mar- 

 morata, plicata bahamensis and scalarina. 



Key to Species. 



I. Length 16 to 28, diam. 1.6 to 2 mm., with 17 or more 

 whorls in truncate specimens; cylindric- tapering ; cor- 

 neous, usually more or less clouded with white; neck 

 long. 



1. Lustreless, nearly smooth in the middle, weakly 

 costulate above and below; whorls flattened. 21 to 

 28 x 2 mm., with 26-28 whorls. 



U. gracillima } no. 152. 



2. Lustreless, with oblique wide-spaced riblets ; 18 x 1.6 

 mm., with 2Qy 2 whorls, truncate, to 20x2 mm., 

 with 29 whorls in entire shells. 



U. cyclostoma, no. 153. 



3. Slightly glossy, smooth or weakly striate; whorls 



