2 9 2 



MOLLUSCA. 



mately bent back in the form of a crosier. The species of 

 Ancyloceras are Jurassic and Cretaceous, ranging from the In- 

 ferior Oolite to the Chalk. 



In the genus Scaphites, the shell resembles that of Ancylo- 

 ceras in consisting of a series of volutions coiled into a flat 

 spiral, and having the last volution de- 

 tached from the others, produced, and 

 ultimately bent back in the form of a 

 crosier. Scaphites differs from Ancylo- 

 ceras in the fact that the volutions of 

 the enrolled part of the shell are in con- 

 tact, instead of being separate as they 

 are in the latter. The produced whorl, 

 also, is rarely of any great length, but 

 is speedily bent back upon itself. All 

 the species of Scaphites are Cretaceous, 

 ranging from the Lower Greensand to 

 the Chalk. 



In the genus Helicoceras the shell is 

 coiled into a turreted spiral, the volu- 

 tions of which are not contiguous. The 

 shell is, also, left-handed or " sinistral." 

 With the exception of a single species 

 from the Inferior Oolite, all the species 

 of Helicoceras belong to the Cretaceous 

 period. 



In the genus Turrilites the shell 

 agrees with that of the preceding in 

 being composed of volutions which 

 pass obliquely round a central axis 

 (fig. 263), so as to form a turreted 

 spiral. The shell is, also, left-handed 

 or " sinistral." In Turrilites, however, 

 the whorls of the shell are in contact, 

 instead of being disconnected as they 

 are in Helicoceras. The genus corre- 

 sponds with Trochoceras in the series of 

 the Nautilida. All the species of Tur- 

 rilites are Cretaceous, ranging from the 

 Gault to the Chalk. 



In the genus Hamites the shell is an 

 extremely-elongated cone, which is bent upon itself more than 

 once, in a hook-like manner, all the volutions being separate. 

 Numerous species of Hamites are known, all of them being Cre- 

 taceous, and ranging from the Lower Greensand to the Chalk. 



Fig. &$. Turrilites 

 catenattu. Gault. 



