=P 
LS) 
AMMONITIDA. 
Gland double, aptychus calcareous. 
Aptychus furrowed externally: Harpoceras, (Eko 
traustes, Oppelia, Haploceras, Scaphites ? 
Aptychus thin, granulated externally: Stephano- 
ceras, Perisphinctes, Peltoceras, Cosmoceras. 
Aptychus thick, smooth and punctate externally : 
Simoceras, Aspidoceras. 
In the absence of positive knowledge as to the true relations 
of the Aptychi with the shells of Ammonites, and until much 
more extensive observations shall have been made, the groupings 
indicated above must be regarded as simply provisional. 
One of the latest authorities on the subject (Prof. Owen, Zool. 
Proc., 955, 1878), regards the aptychi as true opercula. 
The following “ genera ” of Aptychi have been characterized : 
TRIGONELLITES, Parkinson. Shelly, divided into two plates 
by a straight median suture; external surface smooth or sculp- 
tured, inner surface marked by erowth-lines. ‘ 
Associated with the round-backed Ammonites, and a single 
specimen with Goniatites. Nearly fifty varieties have been 
described. 
Meyer considered them bivalve shells, and described them 
under the name of Aptychus; Deslongchamps, with the same 
impression, called them Munsteria ; W@Orbigny thought them 
plates of cirripeds,and Deshayes believed them to be the gizzards 
of Ammonites; Coquand compared them with Teudopsis, and 
they certainly resemble in some degree that genus, as well as 
Beloteuthis, Belemnosepia, ete. 
ANAPTYcHUS, Oppel. Horny and flexible, in a single piece. 
Associated with the Arietes group of Ammonites. 
The classification of the Ammonitide, and particularly of the 
genera dismembered from the old genus Ammonites, is involved 
in much confusion, partly in consequence of the selection by 
several sy Seoul aniaie of different generic characters as of primary 
importance throughout the group, partly owing to the instability 
of some of the most obyious characters. Surface ornamentation 
and even form are now known to change with age; and on this 
account the following scheme of classification of the genus 
Ammonites, elaborated by von Buch and d’Orbigny, is no longer 
available for the discrimination of the several thousand described 
species. As examples of the extent to which naturalists have 
been misled by these mutable characters, it may be mentioned 
that A. splendens, from the greensand of Cambridge, England, 
according to Mr. Seeley, includes fourteen other so- called species 
from the same bed. 
