26 



GEOLOGICAL MEMOIRS. 



they all belong to one species (P. perversum), they serve to establish 

 (from the position of the siphuncle and the slight curvature of the 

 shell) an evident transition from Phragrnoceras to Gomphocevas. 



Fig. 11. 



Phragmoceras callistoma, 

 Bairande. 



Phragmoceras Loveni, 

 Barrande. 



The Bohemian species of 

 Phragmoceras have but 

 little ornament on the shell. 

 They are chiefly character- 

 ized by the general form 

 and the manifold conditions 

 of the mouth of the shell. 

 Thus P. callistoma (fig. 11) 

 has three notches on each 

 side of the larger aperture 

 of the mouth ; and P. Lo- 

 veni (fig. 12) has only two notches on each side. 



9. Ascoceras. — The form of this singular Cephalopod (fig. 13) pre- 

 sents some slight analogy to PtycJioceras among the Ammonitidce. 

 In Ascoceras the shell is bent quite back 

 upon itself, as in Ptychoceras. In the 

 former, however, the chambered portion 

 (fig. 13 y y 7) is very short, and con- 

 tains only 3-7 chambers, and the reflected 

 portion, instead of merely running back 

 parallel to the body-chamber, n /3, lies 

 close to it, and indeed is recessed in it ; so 

 that the whole forms a cylindrical body, 

 covered by one shell, instead of there 

 being two distinct and parallel branches, 

 as in PtycJioceras. The shell, covering 

 at the same time both air-chambers and 

 body-chamber, hinders one from recog- 

 nizing the nature of this fossil so long as 

 the shell is perfect ; but when this is 

 partly removed, the two enclosed parts 



Fig. 13. 



Ascoceras. 

 <i, outline. 



6, transverse section. 

 a (3, the body-chamber. 

 y y y, the air-chambers. 

 S, place of the siphuncle. 



can be distinguished. 



The siphuncle, passing from the body- 

 chamber to the air-chambers, can be easily 

 seen at the extremity of the fossil (S) ; 

 still it was not possible to detect how the 

 several chambers communicated one wdtli another. The mouth (a) 

 is always nearly circular and without contraction, as in Orthoceras. 

 Nearly all the species have a somewhat ornamented shell, which, 

 together vdth the difference of general form, serves for specific di- 

 stinction. They are generally small in size, not exceeding 5-10 

 centimetres in length. 



10. Goniatites. — This genus, the solitary representative of the 

 Aminonitidce, has furnished at least six species, all of which belong to 

 the Nautilini group of MM. Saudberger. I have already pointed 

 out* the great analogy existing between these Goniatites and the 

 Nautili of my Stage E. It is also worthy of remark that all the 

 * [Quart. Journ. Geol. Soc. vol. x. Miscell. pp. 7 & 8. — Ed.] 



