MODERN CLASSIFICATION. 
2G1 
grows near the umbilicus, and tbe sides and ventral area are encircled by small fine 
undivided lines of growth. 
Genus Peltoceras, Waag. — 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 
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, ¥hil. Peltoceras Eugenii, Rasp. 
— Arduennense, d'Orb. — annulare, Rein. 
— Constant!, d'Orb. — Williamsoni, Phillips. 
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 different developmental series of the genus 
to their root, we are carried to some form of Perisphindes wherefrom the genus takes 
1 ' Palseontologia Indica,' pp. 75—89, and Pls> XV— XVII. 
