268 TRIGONELLITES. 



are generally supposed to be opercula ; if so, the}?- were probably 

 secreted b}' the disk or hood, which, formed by the coalescence 

 of the two dorsal arms, closes the aperture of the recent Nau- 

 tilus, and corresponds to the velamentous arms of the Argonaut : 

 but if the Ammonites were dibranchiates allied to Spirula 

 that is having internal shells, they could not have possessed 

 opercula. 



Prof. Waageri 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, Lyttoceras, Arcestes. Pinnoceras, Trachy- 

 ceras. 



B. Nidamental gland with an Aptychus. 



1. Gland simple, not divided. 



Aptychus horny : Arietes, JEgoceras, Amaltheus. 

 Aptychus calcareous: A. nutnida. Coq. (shell un- 

 known). 



2. Gland double, aptychus calcareous. 



Apt} T chus furrowed externally : Harpoceras, (Eko 

 traustes, Oppelia. Haploceras, Scaphites ? 



Aptychus thin, granulated externally: Stephano- 

 ceras, Perisphinctes, Peltoceras, Cosmoceras. 



Aptj^chus 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 group- 

 ings indicated above must be regarded as simply provisional.. 



The latest authority 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 



