77 



Bath. It is somewhat of a discoidal, or rather of a plano-concave form, 

 and is in general from two to three inches in breadth. Its spiral convo- 

 lutions, which are from four to five in number, are strongly carinated, in 

 the direction of the turns, on nearly the middle of the upper side, and 

 become slightly grooved towards the next inner turn. On the under 

 side, they are of a roundish form, and marked with slight but frequent 

 oblique rugae ; the turns being so disposed as to form, in the under part, 

 a tolerably smooth funnel-formed cavity. In one species, distinct nodu- 

 lar projections exist on the lower side of the spiral turns. 



None of the specimens which I possess are in sufficiently good preser- 

 vation to allow me to speak decidedly respecting the aperture of this 

 fossil. I am only enabled to observe, that none of the preceding whirls 

 project into the hollow of the next; as, from my specimens being frac- 

 tured in several places, I am able to perceive that the ridge on the 

 upper part of the shell is a solid external projection, which no ways 

 affects the circular cavity of the shell. Hence, there is reason to con- 

 clude, that the mouth is round, and not diminished by the projection 

 into it of the preceding whirl, as takes place in the planorbiles. 



This fossil was first described by Mr. Walcot in these words: " De- 

 pressed, volutions three, a sharp ridge runs in the centre of the upper 

 surface of the volutions. Lime-stone." Descriptions and Figures of Petrifac- 

 tions, Sfc. Fig. L. and JLVI. 



As far as I am able to judge, from such parts as have not suffered 

 injury, or are riot concealed by the adhering matrix, the fossil repre- 

 sented Plate VI. Fig. 7 and 8, is another species of the same genus, in 

 which the preceding fossil should be placed. It is discoidal; but the 

 central projecting termination of the spire is elevated in a peculiar man- 

 ner, so as at first sight to give it the appearance of being detached 

 from the next turn. On closer examination, this is, however, found 

 riot to be the case ; since a connection is formed by an expansion from 

 the next turn, the rugae of which are continued very closely and regu- 



