42 



Guide to the Invertehrata. 



GALLERY like Crioceras, but at the last whorl the shell is straightened 

 out at a tangent to the coiled part, and after attaining a con- 

 siderable length in a straight course it bends over abruptly in 

 the direction of the coiled part, as in ScapJiites. Crioceras and 

 Ancyloceras are variously and elegantly ornamented with ribbing, 



Table- 

 case 63. 



Fig. 78. — Ancyloceras Matheronianum,WOv\). Neocomian. 



spines, and tubercles. Crioceras extends from the Neocomian to 

 the Upper Greensand, Aneyloceras (Fig. 78) from the Inferior 

 Oolite to the Chalk. 



Section II. — This section • includes Ammonites of flat, disc-like 

 form, with Ceratites-\\kQ sutures and deeply embracing whorls. 



Fig. 79*. — Aptychus of 

 an Ammonite detached. 

 Jurassic. 



Fig. 79rt. — Ammonite with aptychus in 

 aperture of shell. Jurassic, 



Sphenodiscus and Buchiceras (Upper Cretaceous) are characteristic 

 genera. Placenticeras (Cretaceous) includes such species as P. 

 Largilliertianum and P. Orhignyanum. 



Many examples of Aptychi are to be seen in the cases with the 

 Ammonites. The Aptychus is the shelly (or in some cases horny) 

 operculum closing the aperture of the shell in those genera of 



