eee ae 
179 
In Hoplites mammillaris (figs. 24, 25) we have another 
example, showing how the remarkable ornamentation of some 
of the shells of this group depends on the development of the 
mouth-border. Here the ridges with their spines and the 
valleys with their smooth surface succeed each other, owing 
to the advance of the calcareous oral band and its temporary 
cessation of growth, producing the remarkable form of shell 
this species assumes (fig. 24). A similar condition of the 
mouth-border has formed the ornamented shell of Hoplites 
Martini (fig. 26), which I have collected in the Lower Green-,. 
sand (Neocomian) from the Atherfield section, Isle of Wight. 
In Lytoceras the mouth is circular and without lateral 
processes, the shell is distinguished, especially among its Liassic 
forms, for the reticulate structure of the test and the prominent 
frills that are developed at intervals, as in Lytoceras fimbriatum 
(fig. 27). These fimbriz originated from a periodic activity 
in the secreting power of the mantle around the mouth-border 
of the species possessing these fimbriated projections. A similar 
structure is likewise found in Lyt. hircinum, from the Jurense 
zone. 
Fic. 27.—Lytoceras fimbriatum, Sow. Fie. 28.—Phylloceras heterophyllum, Sow. 
In Phylloceras the involute shell of this group is marked with 
true forward inclined lines of growth, indicating the presence of 
a ventral process at the abdominal side of the mouth-border, and 
this appears to have been the case in the only specimen with a 
fan-shaped dwelling-chamber, Phylloceras heterophyllum (fig. 28), 
