THE CEPHALOPODA. 
187 
even as broad as it is deep. The lateral saddle rises far above all the others, and stands 
upon the ground of the upper lateral, in general twice the height of the siphonal saddle. 
l^hQloiDer lateral \?> likewise much broader than deep ; and the cohmellar saddle \% so small 
that it does not reach to half the height or breadth of ihe, lateral saddle. This remarkable 
disposition of the lobes is constant in all the species, and occurs in no other group. 
The Ammonites of this family are in general found in great numbers together. 
Whole strata are sometimes composed of them ; and they often grow to a very large size, 
from one to two feet in diametei". They are exclusively and only special to the Lias 
formation, and particularly in the lower beds thereof, where wq find many different 
species. Tt is still remarkable that this group stands wholly isolated from all the others. 
The following are typical forms, and for the structure of the foliations of the septa see 
PI. HI and IV. 
Arietites Bucklandi, Sow, 
— Conybeari, Sow. 
— bisulcatus, Brug. 
Arietites rotiformis, Sow (fig. 50). 
— stellaris, Sow. 
— semicostatus, Young. 
n. Ealciferi. — In this group it appears that the mantle which formed the 
lobes and radii had been capable of a certain amount of pliability, 
for all rigidity among these forms vanishes. The extremely rami- 
fied lobes surprise us through their digitations, always more or less 
dependent, by which the lobes in their deepest part appear 
not only pointed, but of considerable breadth, scarcely narrower 
than at their mouth. The less incised are especially flat and lie 
almost all at the side of the lateral saddle, in a line behind one 
another, which is almost also the radius of the whorl. The siphonal 
Fig. 54. — Harpoceras ser^enti- 
lobe, much shorter than the upper lateral, pushes the pointed ends num, Schioth. 
of both its arms obliquely against the lateral, so that both arms diverge considerably, and 
its walls do not ascend vertically, but obliquely towards the siphonal saddle. (Eig. 54.) 
When the shell is complete the radii and folds upon the sides are in the highest 
degree fine and delicate. It is the external layer of the shell, which fills in all uneveness 
between the thickness and divisions of the folds of the under layer. All these folds and 
radii 'first bend forward, then turn backward, and near the back again turn towards the 
mouth opening, so that a well-marked, strongly-bent sickle is formed thereby. The inner 
border of the whorl is always blunted by an especial sharp plane surface. The siphonal 
area in most cases rvms out into a sharp ridge, which consists of the siphuncle. This 
family is extremely rich in species, most of which are special to the Upper Lias, some 
are found in the Middle Lias and others in the Lower Oolite. The Falcifers never occur 
in the upper members of the Oolitic series. The following are good types of the genus 
Harpoceras, which comprehends the Falciferi, von Buck. 
