AIPOCERAS COMPRESSUM. 



117 



The septa are moderately distant from each other, indicating shallow chambers. 

 The sutures are very slightly curved backwards upon the sides of the shell, 

 bending a little forwards on the peripheral margin, and crossing the periphery with 

 a very slight backward curvature. 



The sutures exposed to view give the following depth for the chambers, 

 reckoned from the body-chamber posteriorly, measurements being taken at about 

 the middle of the lateral area : 



1 .... . 11-0 mm. 



2 



3 



4 



5 



6 



7 



8 



The siphuncle is ventral. 

 The test is perfectly smooth. 



16-0 

 15*5 

 14-0 

 12-5 

 11-0 

 9-5 

 8-0 



Dimensions. 



Diameter of shell . . . . 



,, umbilicus 



Height of whorl on body-chamber at about 65 mm. 

 from its base . . . . 



Thickness at edge of umbilicus near base of body- 

 chamber . . . . 



Large specimen from Clane in 

 Science and Art Museum, Dublin. 



170 mm. 



50 



84 



Affinities. — The distinctly wedge-shaped whorl and the compressed form of the 

 shell in this species distinguish it completely from Aipoceras gibberosum, de 

 Koninck, sp., the only one with which it can be compared. 



Bemarlcs. — These evolute forms of the Solenocheilidse are singularly inte- 

 resting, because they supply one of the links in the chain that connects the 

 straight shells of the Carboniferous seas and those which are curved, or partially 

 curved, with the completely involute oues. Their relationship with Asi/mptoceras 

 and iSolenocheilus, so far as the form of the shell can dictate it, seems to be clearly 

 made out. The triangular section of the whorl, the ventral position of the 

 siphuncle, and the nature of the septa are similar, except in minor particulars, to 

 those of the genera named. It is only in the matter of uncoiling that they stand 

 alone among the Solenocheilidae. 



The same connection between simply curved and evolute shells and involute 



