MANTELLICERATID®. 109 
in diameter, but usually another line of minute tubercles appears on what 
are to become the umbilical shoulders, and lateral costze and a lateral 
zone begin to appear. This occurs before or contemporaneously with the 
appearance of another line of tubercles on the ventral costz close to the 
already existing ventral lines. In the next stage a third line appears on 
the venter next to the last developed, and usually nearer to them than to 
the ventro-lateral line of primitive tubercles. At or about the same time 
still another line of tubercles appears on the lateral costee midway between 
the two lateral lines. This stage of twelve tubercles is sueceeded by a 
fourteen-tuberculated stage, and in some cases by a sixteen-tuberculated 
stage, through the generation of additional lines of tubercles on the venter. 
During these later stages the form of the volution changes, losing entirely 
its coronate aspect. The venter becomes more and more elevated and 
rounded, the costee more prominent, and the tubercles are apt to become 
more and more equal in size, so that eventually in some species it is diffi- 
cult to pick out the primitive lateral tubercles. The stage of neanic age, 
in which there are six lines of prominent spines, is exactly similar to the 
young of Pseudaspidoceras footeanum as figured by Stoliezka in his Fossil 
Cephalopods of the Cretaceous Rocks of Southern India (pl. 52). 
This genus includes the species described below. 
DovuvVILLEICERAS MAMMILLARE (d’Orbigny). 
Ammonites mammillaris VOrbigny (pars). 
Assuming that the young of d’Orbigny’s species on pl. 72 represents 
the mammillatus of Schlotheim, it is evident from the collections in the 
Museum of Comparative Zoology that his supposed mammillaris given on 
pl. 73 is a distinct species. The young have the same crowded costz and 
aspect of this species only throughout the neanic stage. In this species 
this crowded condition of the costee and the development of additional 
rows of crest-like tubercles on the venter seem to continue indefinitely. 
In some specimens that can hardly be considered as anything more than 
sporadic varieties, there are one or two:additional lines of tubercles gen- 
erated within the ventral channel, and in our shell a line of faint tubercles 
finally appear in the middle of this channel, thus imitating transiently the 
ornamentation of a distinct family, viz, the Mammitide. 
Locality: Cherbourg, St. Croix, ete. 
Age: Albian. 
