'2li) THTSOA, (iONlOCERAS. ETC. 



Tretoceras, Salter. 

 . Founded ou 0. hisvphonatiun^ Sowb., fi'om the Oaradoc sand- 

 stone (Silurian), Brit., in which the sei)ta are apparentl}' perfo- 

 rated b}' two siphuncles ; one of whicli is a deep lateral cavity 

 continuous with the termiinal chamber — the cavity affecting at 

 least seven of the up|)erinost septa, if not the whole. 

 T. BTsiPHONATUM. Sowb. PI. 97. fig. 481. Silurian. Walea. 



Thisoa, Montf. 

 Shell ovate-elongate, cu(nimber-shaped ; apparently two siphons 

 running parallel the whole length of the shell, one of which trav- 

 erses a sort of narrow lateral cavity ; there are also a number 

 of false sii)hoiis or holes, whicli do not extend the entire length 

 of the shell. 

 T. srvHONALis*. Seri-es. PI. 104, figs. ,571, 672. Jurassic. France. 



Gonioceras, Hall. 



Shell rtattened. with extremely salient angles ; septa sinuous ; 

 sectif)n of shell, an extended ellipse with projecting angles ; 

 siphuncle ventral. 

 G. ANCEPS, Hall. PL 97. ligs. 488. 484. L. Silurian. 



Colpoceras, Hall. 

 This is probably only a siphon of one of the larger species of 

 Orthocerata. 

 C. VTK(^ATiTM. Hall. I'l. 97. tig. 485. L. Silurian. New York. 



Dictyoceras, Eichw. 

 Is jirobalily an Orthoceras covered by a l)ryozoan or coral. 



Genus CLINOCERAS, Masoke. 



Shell conical (allied to Lo.Toceras^ McCoy), the sii)huncle side 

 straight, the othei's more or less curved ; a consti'iction below 

 the body-chamber. Sei)tal border with an obtuse-angled saddle 

 on the siphuncle side, with gently rounded lobes and two slightly 

 marked lateral saddles. 



C. T»ENS. Mascke. PI. 105. fig. 587. Erratic; L. Silurian blocks. 

 Prussia. 



