GENERA OF AMMONITID^. 83 



1. Aiotychus thin ; body-chamber short ; ■\ Harpoceras. 



aperture with a falciform margin > -,-'.' 



^ . , , . , • , i Jurassic, 



prolonged above into a point. ; 



2. Aptychus thick ; body - chamber \ 



short, with a falciform margin, f Oppelia. 



prolonged above into a rounded ( Jurassic and Cretaceous, 

 appendage. ) 



3. Body- chamber short; a groove or ^ 



dilatation near the aperture ; the ( Ha-ploceras. 



aperture with lateral and superior ( Jurassic and Cretaceous, 

 extensions, the latter rounded. ) 

 h. Aptychus thin ; granulated externally. 



1. Body - chamber long ; the mouth ) 8te])lmnoceras. 



simple, or with lateral extensions. ) Jurassic and Cretaceous. 



2. Body-chamber long; the aperture -| Perisphinctes. 



constricted by a furrow, or fur- j^^^^^^^^ ^^^^^ Cretaceous, 

 nished with lateral extensions. ; 



3. Peltoceras. Jurassic. 



4. Body-chamber short ; the aperture \ r< 



. ■' , . , .' . , , 1 / Vosmoceras. 



simple or provided with lateral V j 



extensions. ; 



c. Aptychus thick, smooth, and externally punctate. 



1. Body -chamber long ; a large um- ^ 



bilicus ; the shell furrowed ; the f Simocercis. 

 margin of the aperture with a ( Tithonic. 



pointed upper prolongation. } 



2. Body -chamber short ; the margin ) Aspidoceras. 



of the aperture generally simple. ) Jurassic and Cretaceous. 



Leaving tlie great genus Ammonites, we have a number of 

 more or less closely allied types to consider, of which only 

 the more important ones can be alluded to here. 



In the genus Crioccras (fig. 473, d) are included forms 

 which resemble the Ammonites in all essential characters, 

 but in which the volutions of the shell are not contiguous. 

 The shell, therefore, is discoidal, with separate whorls, thus 

 corresponding with Gyroccras amongst the series of the 

 NautilidcB. All the known species of Crioceras belong to 

 the Cretaceous period, ranging from the Lower Greensand to 

 the Gault. 



Choristoceras, of the Trias, resembles Crioceras in shape, 

 but the septa and sutures have the form characteristic of 

 Ceratites. 



In the genus Toxoccras the shell is simply arcuate, or bent 



