MODERN CLASSIFICATION. 



261 



grows near the umbilicus, and the sides and ventral area are encircled by small fine 

 undivided lines of growth. 



Genus Peltoceras, Waa^. — Shell flat, discoidal, with a very large umbilicus ; whorls 

 with strong straight ribs, which are mostly provided with two or three rows of spines in 

 adult specimens ; siphonal side more or less flattened, or even excavated, the ribs passing 

 over it or disappearing before they reach it. Inner whorls with strong, sharp, mostly 



Fig. 181. — Peltoceras athleia, Phillips. 



dichotomate ribs, sometimes undivided, outer whorls with strong, blunt, bi-tuberculous ribs, 

 which pass over the siphonal area. The lobe-line highly ramified ; siphonal lobe large, 

 with symmetrical divergent branches ; superior lateral widely separated from the siphonal, 

 and forming a broad external saddle ; lower lateral small and imperfectly developed, or even 

 wanting ; sometimes the ribs are in part replaced by spines. The amount of involution 

 is small ; the length of the body-chamber not known ; and the mouth-border has large 

 lateral auricles developed from its sides. The range of the species is limited in this 

 country to the Oxfordian strata. 



Peltoceras athleta, Fhil. 



— Arduennense, (TOrb. 



— Constanti, (VOrh. 



Peltoceras Eugenii, Rasp. 



— annulare. Rein. 



— Williamsoni, Phil 



Some interesting forms of this group, collected by the Indian Geological Survey, have 

 been figured and described by Dr. Waagen,^ who observes "that the general type of the 

 shell is modified in different species in several directions, such as that the spines are 

 entirely wanting, or the ribs nearly entirely replaced by corresponding spines ; but the 

 general type of the genus can, nevertheless, even in such forms, be easily recognised. 



" There is no doubt that, if we follow up the diff'erent developmental series of the genus 

 to their root, we are carried to some form of Perispldnctes wherefrom the genus takes 



1 ' Palaeontologia Indica,' pp. 75—89, and Pis. XV— XVII. 



