AMMONITID^. 61 



Fischer, who phices it between the Dibranchiata and Tetrabran- 

 chiata. The principal cliaracter which the AmmonitidjB possess 

 in common with the former is tlie early development of the shell, 

 the initial chamber being without cicatrice ; but, on the other 

 hand, the possession of an external shell is abundantl}^ indicated 

 by its exterior ornamentation, by its opercular plates and by 

 other characters. I prefer to consider them tetrabranchiates — 

 an opinion powerfully supported b}^ Prof. Bichard Owen, so 

 lately as 181S. 



The Ammonitidte became essentially extinct towards the close 

 of the secondary- period, although a few forms are now referred 

 to the tertiary. The geological position of the genera may be 

 thus indicated : 



Arcestes, Dldymites, Lobites, Pinacoceras, Ptychites, Trachy- 

 ceras, Tropites are exclusively Triassic. 



Arietites, Harpoceras, (Ekotraustes, Oppclia, Peltoceras, 

 Stephanoceras, Simoceras, etc., are Jurassic. 



Acanthoceras, Olcostephanus, Schloenbachia, Stoliczkaia, etc., 

 are Cretaceous. 



Sageceras is Permian and Triassic. JEgoceras is Triassic and 

 Liassic. Amaltheus, Lytoceras, Phylloceras, occur in Triassic, 

 Jurassic and Cretaceous beds. Aspidoceras, Cosmoceras, Hap- 

 loceras and Perisphinctes are found in the Jurassic and 

 Cretaceous. 



The aptychi or so-called opercula of Ammonites (ii, 33) are 

 constantly found associated with (and generally within the aper- 

 ture of) the shells of some of the groups. They are horny or 

 shell}^ plates, and have been generally supposed to be opercula; 

 if so, they were probably secreted by the disk or hood, which, 

 formed by the coalescence of the two dorsal arms, closes the 

 aperture of the recent Nautilus, and corresponds to the velamen- 

 tous arms of the Argonaiit. If the Ammonites were dibranchiates 

 allied to Spirula — that is having internal shells — they could not 

 have possessed opercula. 



Prof. Waagen has adopted the theory first suggested by 

 Keferstein and advocated by Zittel, that the aptychi were con- 

 nected with the nidamental gland ; and he has grouped the 

 family according to the presence absence or peculiarities of 

 these bodies as follows : 



A. Nidamental gland without solid integument or Aptyehus : 



Phylloceras, Lytoceras, Arcestes, Pinnoceras, Trachy- 

 ceras. 



B. Nidamental gland with an Aptyehus. 



1. Gland simple, not divided. 



Aptyehus horny : Arietes, J^]goceras, Amaltheus. 

 Aptyehus calcareous : A. numida^ Coq. (shell un- 

 known). 



