268 trigonellttp:s. 



are generally supposed to be operciila ; if so, they were probably 

 secreted b}'^ the disk or hood, M'hich, 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, the}' could not have possessed 

 opercula. 



Prof. Waagen has adopted the theory tirst 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. al)sence or peculiarities of 

 these bodies, as follows : 



A. Nidamental gland without solid integument or Aptychus: 



Phyllocera>i. Lijtfoceras, Arcestes. Pinnoceraa^ Ti-achy- 

 ceras. 



B. Nidamental gland with an Aptychus. 



1. Gland simple, not divided. 



Aptychus horny : Arietes, ^goceras, Amaltheus. 

 Aptychus calcareous : A. numida, Coq. (shell un- 

 known). 



2. (Jland double, aptychus calcareous. 



Aptychus furrowed externally : Harpoceras, (Eko 

 trmistes, Opjielia, Haploceras. Scaphites ? 



Ai)tychus thin, granulated externally : Stephano- 

 ceras, Perisjyhinctes. Peltoceras. Cosmoceras. 



Aptychus thick, smootli 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 he 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 



