62 AMMONITID^. 



2. Gland double, aptyclius calcareous. 



Aptyclius furrowed externally : Harpoceras, ffiko 



traustes, Oppelia, Haploceras, Scaphites ? 

 Aptyclius thin, granulated externally: Stephano- 



ceras, Perisphinctes, Peltoceras, Cosmoceras. 

 Aptyclius 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 growth-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 Aptyclius; Deslongehamps, with the. same 

 impression, called them Munsteria ; d'Orbign}^ thought them 

 plates of cirripeds, and Deshayes belicA^ed them to be the gizzards 

 of Ammonites ; Coquaiid compared them with Teudopsis, and 

 they certainly resemble in some degree that genus, as well as 

 Beloteuthis, Belemnosepia, etc. 



Anaptychus, Oppel. Horny and flexible, in a single piece. 

 Associated with the Arietes group of Ammonites. 



The classification of the Ammonitidffi, and particularly of the 

 genera dismembered from the old genus Ammonites, is involved 

 in much confusion, partly in consequence of the selection by 

 several sj'stematists of different generic characters as of primary 

 importance throughout the group, partly owing to the instability 

 of some of the most obvious 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 greeiisand of Cambridge, England, 

 according to Mr. Seeley, includes fourteen other so-called species 

 from the same bed. 



