AMMONITID2. 61 
Fischer, who places it between the Dibranchiata and Tetrabran- 
chiata. The principal character which the Ammonitidz possess 
in common with the former is the early development of the shell, 
the initial chamber being without cicatrice ; but, on the other 
hand, the possession of an external shell is abundantly indicated 
by its exterior ornamentation, by its opercular plates and by 
other characters. I prefer to consider them tetrabranchiates— 
an opinion powerfully supported by Prof. Richard Owen, so 
lately as 1878. 
The Ammonitide 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, Didymites, Lobites, Pinacoceras, Ptychites, Trachy- 
ceras, Tropites are exclusively Triassic. 
Arietites, Harpoceras, (£kotraustes, Oppelia, Peltoceras, 
Stephanoceras, Simoceras, etc., are Jurassic. 
Acanthoceras, Olcostephanus, Schleenbachia, Stoliczkaia, etc., 
are Cretaceous. 
Sageceras is Permian and Triassic. Aigoceras 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 
shelly 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 ofthe Argonaut. Ifthe 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 Aptychus: 
Phylloceras, Lytoceras, Arcestes, Pinnoceras, Trachy- 
ceras. 
B. Nidamental gland with an Aptychus. 
1. Gland simple, not divided. 
Aptychus horny: Arietes, Aigoceras, Amaltheus. 
Aptychus calcareous: A. numida, Coq. (shell un- 
known). 
