AMMONITID-ffi. 



291 



SECTION D. Back channelled. 



7. Dentati . . . Oolitic and Cretaceous. Ex. A. Jason. 



8. Gemmati . . . Trias. Ex. A. Aon. 



SECTION E. Back squared. 



9. Flexuosi . . . Cretaceous. Ex. A. radiatus. 



10. Compressi . . Cretaceous. Ex. A. Beaumontianus. 



11. Armati . . . Oolitic. Ex. A. perarmatus. 



12. Angulicostati . Oolitic and Cretaceous Ex. A. Milletianus. 



SECTION F. Back round. 



13. Capricorni . . Lias. Ex. A. planicostatus. 



14. Heterophylli . . Oolitic. Ex. A. heterophyllus. 



15. Ligati .... Cretaceous. Ex. A. Mayorianus. 



16. Planulati . . . Oolitic. Ex. A. annulatus. 

 \1~jCoronati . . . Oolitic. Ex. A. Humphresianus. 



\%~ Macrocephali . . Oolitic and Cretaceous. Ex. A. microstoma. 



19. Globosi . . . Trias. Ex. A. globus. 



20. Fimbriati . . Jurassic and Cretaceous. Ex. A. sub-fimbriatus. 



In the genus Crioceras are included forms which resemble 

 the Ammonites in all essential characters, but in which the 

 volutions of the shell are not contiguous. The shell, there- 

 fore, is discoidal, with separate whorls, thus corresponding 

 with Gyroceras amongst the series of the Nautilida. All the 

 known species of Crioceras belong to the Cretaceous period, 

 ranging from the Lower Greensand to the Gault. 



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

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

 represents Cyrtoceras in the series of the Nautilidee. The 

 species of Toxoceras range from the lower Oolites to the Gault, 

 but the genus is characteristically Cretaceous. 



In the genus Ancyloceras (fig. 262) the shell at first re- 



Fig. 262. Ancyloceras Mathe, 



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 ulti- 



