AMMONOIDEA. 



867 



is simple and without side-lappets. The surface-sculpture is similar 

 to that of Stephanoceras. All the species of Cozloceras are found in 

 the Lias, two well-known forms being 

 C. (Amftionites) annulatum (fig. 793), 

 and C. (Ammonites) commune (fig. 



794). 



In Cosmoceras (fig. 795) the shell 

 is discoidal and the surface-ribs are 

 bent forwards, and often have tuber- 

 cles or spines developed along their 

 external ends. The aperture of the 

 shell is furnished with long lateral 

 extensions, which may disappear with 

 age. The species of Cosmoceras are Jurassic and Lower Cretaceous. 



In Perisphi?ictes (fig. 796) the shell is discoidal, and usually widely 

 umbilicated, with a rounded external border. The surface-ribs 

 divide, each, near the siphonal margin, into two or more branches. 

 The body-chamber extends over two-thirds or the whole of the last 

 whorl, and the aperture is simple, with lateral extensions or auricles. 



Fig. 795. — Cosmoceras (Ammonites) 

 Jason, reduced in size. Jurassic 

 (Oxford Clay). 



Fig. 796. — Perisphinctes (Ammonites) 

 Martinsi. Inferior Oolite. 



Fig. 797. — Hoplitcs (Ammonites) 

 falcatus. Cretaceous. 



A very large number of species of this genus are known, ranging 

 from the Inferior Oolite to the Lower Cretaceous. 



In the genus Hoplites (fig. 797) the shell differs from that of 

 Perisphinctes in being involute, with the umbilicus narrow, and 

 the whorls high. The surface-ribs spring from tubercles near the 

 umbilical margin, which divide as they proceed outwards, and are 

 generally interrupted by a broad furrow along the line of the external 

 border of the shell. The species of this genus are characteristic of 

 the Cretaceous system. 



In Acanthoceras the shell (fig. 798) is discoidal and umbilicated, 

 and the sculpture consists of strong, straight, undivided, or divided 



