Clarke — The Naples Fauna. 119 



to the lateral slopes instead of being distinctly ventral. Further, the shell 

 has been represented and described .as distinctly umbilicate at large size. 



The species is of rare occurrence in the Genesee and Naples regions and 

 appears only in very diminutive form. The described specimens are recorded 

 as found along the lake Erie shore and it is from this region that our speci- 

 mens, also, have been obtained. 



Adult characters. At maturity the species seems seldom to surpass a 

 diameter of 25 mm. It is discoidal in form, with a broadly convex or flat 

 venter and the umbilicus is closed. AVe shall presently observe that the 

 obliteration of umbilication is affected only in the final volution, so that the 

 removal of the shell of the outer whorl will expose the inner volutions and 

 give the shell an umbilicate aspect. About the ventral periphery runs a pair 

 of low revolving grooves, one on either side, without which are rather low 



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hyponomic ridges. 



The ornament at this stage consists of fine and crowded or, sometimes, 

 sharp and more distant concentric lines which curve broadly backward over 

 the lateral slopes and then sharply forward on the hyponomic grooves and in 

 a deep linguiform festoon backward upon the venter. 



The septation is different from that of Torn, uniangulare only in the 

 greater prominence and isolation of the lateral lobe. 



Immature Growtlv-stages. The Protoconbh. Although we have not been 

 able to isolate the embryonic shell, it will be seen from our drawing both of 



97 98 



Figures 97. 98. Tornoceras bicostatum. Fig. 97. An external impression of the 

 inner whorls, showing the very small protoconch.the gradual expansion and diminution 

 of the nepionic shell and the distant subaqual varices of the neanic stage. Fig. 98. 

 Lateral view of the protoconch, nepionic and early neanic whorls. The surface doe* 

 not retain the fine ornament shown in fig 94. snd the varix on the second wborl is 

 probably a labial ridge, x 25. 



lateral and sectional views that it is exceedingly small ; thus similar to, but 

 even more extreme in this respect than Torn, uniangulare. It may to some 

 degree, be concealed by the broadening of the first whorl but it is not clear 

 that this is the case. 



