84 



CEPHALOPODA. 



like a horn, and is never spirally rolled up ; so that this 

 genus represents Cyrtoccras in the series of the Nautilidce. 

 The species of Toxoceras range from the Lower Oolites to the 

 Gault, but the genus is characteristically Cretaceous. 



In the genus Ancyloceras (fig. 471) the shell at first re- 





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Fig. 471. — Ancyloceras Matheronianus. Gault. 



sembles that of Crioceras, consisting of several volutions 

 which are coiled into a flat spiral, but which are not in 

 contact with one another. The shell differs from Crioceras, 

 however, in the fact that the last volution is produced at a 

 tangent, and is ultimately bent back in the form of a crosier. 

 The species of Ancyloceras are Jurassic and Cretaceous, rang- 

 ing from the Inferior Oolite to the Chalk. 



In the genus Scaphitcs (fig. 473, e) the shell resembles 

 that of Ancyloceras in consisting of a series of volutions coiled 

 into a flat spiral, and having the last volution detached from 

 the others, produced, and ultimately bent back in the form 

 of a crosier. Sccqjliites differs from Ancyloceras in the fact 

 that the volutions of the enrolled jDart of the shell are in 

 contact, instead of being separate as they are in the latter. 

 Tlie produced whorl, also, is rarely of any great length, 

 but is speedily bent back upon itself. All the species of 

 Sccqihites are Cretaceous, ranging from the Lower Greensand 

 to the Chalk. 



In the genus Helieoceras the shell is coiled into a turreted 

 spiral, tlie volutions of which are not contiguous. The shell 

 is also left-handed or " sinistral." Witli tlie exception of a 

 single species from the Inferior Oolite, all tlie species of 

 Helieoceras belong to the Cretaceous period. 



In the genus Turrilitcs the shell agrees with that of the 

 preceding in being composed of volutions wliich pass ob- 



