NAUTTLTD^. 53 



chamber — the cavit}' affecting at least seven of the nppermost 

 septa, if not the whole. 



THISOA, Montf. Shell ovate-elongate, cucumber-shaped ; appar- 

 ently two siphons running parallel the whole length of the shell, 

 one of which traverses a sort of narrow lateral cavity ; there are 

 also a number of false siphons or holes, which do not extend the 

 entire length of the shell. T. siphonalis^ Serres (xxix, 82, 83). 

 Jurassic; France. 



GONiocERAS, Hall, 184T. Shell flattened, with extremely salient 

 angles; septa sinuous; section of shell, an extended ellipse with 

 projecting angles; siphuncle ventral. G. anceps, Hall (xxx,94, 

 95). L. Silurian ; IST. America. 



COLPOCERAS, Hall. This is probably only a siphon of one of 

 the larger species of Orthocerata. G. virgatum^ Hall (xxx, 96). 

 L. Silurian ; New York. 



DiCTYOCERAS, Eichw., 1859. Is probably an Orthoceras covered 

 by a bryozoan or coral. 



TREMATOGEEAS, Whitfield. Tube, septa and siphuncle like 

 Orthoceras, but with a line of elongated, raised tubercles along 

 one side of the shell, which have formed perforations at certain 

 stages of growth, probabl}^ confined to the outer chamber as 

 openings, which were closed as the animal extended the shell, 

 and before the septa opposite them were formed. Type T. Ohio- 

 ense, Whitfield. Upper Helderberg group, Ohio. 



[PoLORTHUs, Gabb, 1861. 

 The aggregated mass of specimens forming the type of this 

 genus was originally referred to Teredo ; subsequently, in de- 

 scribing the genus, Mr. Gabb referred it to Yermetidse, and in 

 18*72 he finally believed it to be a cephaiopod connecting the 

 Orthoceratidee with Beatrices. The aggregate character, the 

 long, narrow, irregular tube, the non-molluscan character of the 

 partitions forbid this determination. I am convinced that 

 Polorthus is not a mollusk, and Beatricea itself is now referred, 

 doubtfully, to the sponges.] 



Clinoceras, Mascke, 1816. 

 Distr. — G. dens, Mascke (xxviii, 13). Erratic L. Silurian 

 blocks ; Prussia. 



♦ Shell conical (allied to Loxoceras, M'Coy), the siphuncle side 

 straight, the others more or less curved ; a constriction below 

 the body-chamber. Septal border with an obtuse-angled saddle 

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

 marked lateral saddles. 



Bactrites, Sandberger, 1842. 

 Syn. — Stenoceras, d'Orb, 1850. 



Distr. — 13 sp. Silurian to Trias. ; Germany, etc. B. gracilis, 

 Sandb. (xxxii, 22). Nassau. 



