MODERN CLASSIFICATION. 257 



assumes in this genus. The mouth-border and the length of the body-chamber is at 

 present unknown. 



The lobe-line is much ramified in Iloplites interruptus, Briig. The siphonal lobe 

 and two principal laterals are about the same length, with numerous branches having a 

 symmetrica] arrangement ; the second lateral is much smaller, and there are several 

 auxiliai'ies. In general the lobe-bodies are slender, the saddles broader than the lobes, 

 the principal lateral is often longer than the siphonal, the second lateral very short, 

 and there are several horizontal or oblique auxiliaries. 



This genus is very characteristic of the Cretaceous period, and its highly ornamented 

 shells form conspicuous groups in the different stages of the Chalk age. 



Genus AcANTHocERAS, Neum. — This new group has been proposed to receive several 

 remarkable Ammonites which were included in the genus Hoplites, a more extended study 

 of them having induced Prof. Neumayr to separate these into a distinct genus, of which 

 the following is a diagnosis : — The shell is wider and more massive ; the irmbilicus more 

 open ; the whorls moderately high ; mouth-border and body-chamber both unknown. 

 The sculpture consists of strong straight ribs, which proceed from the suture-line along 

 the sides, or strong constantly increasing ribs, with a number of tubercles, which adorn 

 the sides and border. The siphonal area presents many differences ; sometimes the 

 median line is channelled, with rows of tubercles on the border, so that the ribs are 



i 



^'- 'I ;j^ 



Fig. 173. Fig. 174. 



Acanlhoc. laticlavium, Sharpe. Acanthoc. Brottiamim, d'Orb. 



interrupted from crossing ; in other species the channel is wanting and the "ribs pass 

 across. Sometimes the tubercles arrange themselves in a series of rows, or pass along 

 the median line and form a kind of knotted keel. The lobe-line presents considerable 

 variety in these highly ornamented shells. The siphonal and principal lateral lobes are 

 about the same size, sometimes the former is much larger than the latter ; and the second 

 lateral is much smaller than the first, and both terminate in single points ; there are 

 several small deep standing auxiliaries. 



All the species are Cretaceous, and the following are typical forms : 



34 



