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, 

 x'eckoued from the body-chamber jjosteriorly, measurements being taken at about 

 the middle of the lateral area : 



1 .... . 11-0 mm. 



2 .... . 16-0 „ 



3 .... . 15-5 „ 



4 .... . U-0 „ 



5 .... . 12-5 „ 



6 .... . 11-0 „ 



7 . . . . . 9-5 „ 



8 . . . . . 8-0 „ 

 The siphuiicle is ventral. 



The test is perfectlj^ smooth. 



Dimensions. 



Large specimen from Clane in 

 Science and Art Museum, Dublin. 



Diameter of shell . . . .170 mm. 



,, umbilicus . . .50 „ 



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



from its base . . . . 84 ,, 



Thickness at edge of umbilicus near base of body- 

 chamber . . . . 55 ,, 



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

 shell in this species distinguish it completely fi'om Aijwceras rjibherosum, de 

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



BemarJcs. — These evolute forms of the Solenocheilidce 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 ones. Their relationship with Asymptoceras 

 and Solenocheilus, so far as the form of the shell can dictate it, seems to be cleai'ly 

 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 Solenocheilidje. 



The same connection between simply curved and evolute shells and involute 



