Nautiloidea. 



25 



Gomphoceras is sometimes ovoid and nearly straight, sometimes Gomplio- 



feebly curved : its aperture is very much constricted and more °^^*'' 



. . • QALL£KY 



or less T-shaped, and its siphuncle a little inflated between the yjj 



septa. This genus, according to our present knowledge of it, -v^all- 



is restricted to the Silurian epoch, numerous examples of both case 7 



forms of the genus occurring in the Silurian rocks of Shropshire 



and Worcestershire. 



Fig. 46. — Gomphoceras. Silurian : Shropshire, a, nearly straight form 

 [ = Gomphoceras as formerly restricted), b, curved form {= Phragmoceras 

 as formerly restricted), s, s, the siphuncle. 



case 7. 



Ascoeeras is a small and somewhat aberrant form. It consists of Ascoceras. 

 two portions, the first, or older, resembling that of an Orthoceras, Wall- 

 having a cylindrical shell, with deep chambers, and a slender, 

 tubular siphuncle, and this stage being succeeded by a second or 

 Ascoceras-stage, after the completion of which the older or Ortho- 

 ceras portion was cast off; hence its rare preservation. In the 

 Ascoceras- stage the shell is sac-like; the septa are fairly regular 

 and close together on one side of the shell, but sweep upwards 

 towards the aperture in a sigmoid curve before they are attached 

 to the other side of the shell. The aperture is simple and open, 

 as in Orthoceras ; but in a closely allied form, Glossoceras, the 

 aperture is partly closed by lobes extending from its margin. 

 Ascoceras occurs in the Ordovician of North America and the 

 Silurian of Europe, by far the greater proportion of the species 

 coming from Bohemia. (See Fig. 47.) 



Poterioceras ranges from the Ordovician to the Carboniferous. 

 It is a pear-shaped shell, the smaller part being chambered and 



