36 



Guide to the Invertehrata, 



GALLERY Permian rocks of Sicily. PtycJiites and Gymnites, from the Alpine 

 ' Trias, are among the best-known genera. Baraelites of the Permian 



69, of Sicily is specially interesting from the fact that its sutural 



characters resemble those of some of the Goniatites; and taking 

 the Ptychitid^ as a whole it is considered that they present a 

 gradational series connecting the Goniatites with the Ammonites. 



Fig. 



—Turrilites catenatus, D'Orb. 

 Gault: Folkestone. 



Fig. 64. — Seteroceras Emerici, D'Orb. 

 Cretaceous . 



Table- 



The principal feature observable in the Amaltheid.e is the heel 



or projecting edge of the outer border (periphery) of the shell, 



_^ 68, which is prolonged beyond the margin of the aperture in the form 



4, 5, 9-11. of a long narrow process. In Amaltheus margaritatus (Middle 



Lias) the keel is prominent and beautifully sculptured, resembling 



Fig. 65. — Baculites anceps, Lamk. Upper Cretaceous. 



the strands of a miniature rope. Cardioceras cordatum (Oxford 

 Clay) is one of the most highly ornamented of Ammonites, having 

 a series of numerous sharp ribs upon the sides of the shell, which, 

 in passing over the periphery, form a series of fine crenulations 

 (Fig. 66). Schlcenbachia varians (Lower Chalk) has strong and 

 knotted ribs. 



