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. 

 The lotver lateral is likewise much broader than deep ; and the columellar saddle \% so small 

 that it does not reach to half the height or breadth of the 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 diameter. They are exclusively and only special to the Lias 

 formation, and particularly in the lower beds thereof, where we find many different 

 species. Tt is still remarkable tliat 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, Brvg. 



Arietites rotiformis, Sow (fig. 50). 



— stellaris, Soiv. 



— semicostatus, Young. 



n. Palciferi. — 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 



, ,, 1111-1, Fig. 54. — Harpoceras serpenti- 



lobe, much shorter than the upper lateral, pushes the pomted ends mm, 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. (Fio-. 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 runs 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. 



