224 AMMONITES. 



Genus AMMONITES, Brug. 



Shell spiral, discoidal, more or less involute ; septa undulated, 

 the sutures lobed and foliated; siphuncle dorsal ; margin of 

 mouth sometimes produced into one or more horns. 



About 700 species. Triassic, Cretaceous. N. and S. America : 

 Europe ; India ; New Zealand. 



A. OBTUSUS. PL 101, fig. 537. Liassic. Lyme Heyi*. 



The young shell differs much from older specimens in the 

 degree of involution and in being less complicated in the suturnl 

 lobes ; even the external ornamentation varies, so that the above 

 number of described species will probably be much reduced 

 upon careful revision. As an example, A. spendenx from the 

 greensand, Cambridge, Eng., according to Mr. Seeley includes 

 fourteen other so-called species from the same bed ; and A. plan- 

 ulatus may include five species. 



Ammonites have been discovered in the high passes of the 

 Himalaya Mountains, over 16,000 feet above the sea. 



The nominal species being so numerous, it has been usual 

 until recently to class them in the following artifical groups : 



A. Dorsal portion of whorls rounded, not keeled. 



1. Fimbriati. Oolitic. A. FIMBRIATUS, d'Orb. PL 101, figs. 538, 531). 



2. Planulati. Jura, Chalk. A. ANNULATUS, Sowb. PI. 102, figs. 540, 541. 



3. Ligati. Cretaceous. A. LIGATUS, d'Orb. PL 102, figs. 542, 543. 



4. Globosi. Alpine Trias. 



5. Heterophylli. Jura, Alpine Trias. A. HETEROPHYLLUS, d'Orb. 



PL 102, figs. 544, 545. 



B. Whorls dorsally flattened. 



6. Capricorn!. Jura. A. CAPRICORNUS, Schloth. PI. 1 02, figs. 546, 547. 



7. Armati. Jura. A. LONGISPINUS, Sowb. PL 102, figs. 548, 541). 



8. Coronarii. Jura, Chalk. A. BLAGDENI, Sowb. PL 102, fig. 550, 551. 



9. Macrocephali. Jura. A. HERVEYI, Sowb. 



10. Compressi. Chalk. A. BEAUMONTIANUS, d'Orb. PL 102, figs. 552, 55:5. 



C. Dorsally channeled. 



11. Dentati. Jura, Chalk. A. MAMILLARIS, Schloth. PL 103, fig. 554, 555. 



D. Dorsally keeled, keel entire. 



12. Arietes. Lias. A. BIPRONS, Brag. PL 103, fig. 556. A. BISUL- 



CATUS, Brug. PL 103, fig. 557. 



13. Falciformi . Jura. A. SERPENTINUS, Scbloth. PI. 103, iigs. 558, 559. 



14. Cristati. Chalk. A. CRISTATUS, Deluc PL 103, fig. 560. 



