60 AMMONITID^. 



situated within the centre, or between the middle and the inner 

 margin, and not funnel-shaped, but tubular and gradually 

 tapering. Distr. — Eocene, Cret. ; Europe, America. 



A very doubtful group, as Conrad includes species having 

 respectively the characters of Aturia and of Nautilus ; the type 

 species is N. orbiculatus, Tuomey. 



PSEUDONAUTiLUS, Meek, 18*76. Differs from Hercoglossa in 

 the septa being provided with well-defined peripheral and anti- 

 peripheral lobes and the siphuncle placed near the outer margin. 

 Nautilus Geinitzi^ Oppel. 



CRYPTOOERAS, d'Orb., 184T. ( Solenochilus, Meek and Worthen, 

 IStt.) Planorbiform ; septa arcuated, without lobes or sinuos- 

 ities ; siphon dorsal. Distr. — 2 sp. Devonian, Carboniferous ; 

 Europe. G. subtuber culatus, d^Orh. (xxxi, 504), 



Beaks op TETRABRANcnrATEs. 

 These are found associated with fossil Nautili and occasionally 

 Belemnites, but never with Ammonites. The upper beaks have 

 been described under the name of Rhyncholites, the lower ones 

 as Conchorhynchus. 



B. Astieriana, d'Orb. (xxxv, 74). 



C. avirostris, Bronn (xxxv, 75). 

 G. Owenii, 'Bronn (xxxv, 76). 



PELTARiON, Deslongchamps. This was formerty believed to be 

 the mandibular armature of tetrabranchiates, consisting of 

 circular or transversely -oval calcareous plates, with rounded 

 anterior and produced and truncated posterior margins. Through 

 the researches of M. Crosse (Jour, de Conch. ^ 3 ser., xv, 57, 

 1875), there is no doubt that these Peltarise are opercula of 

 fossil species of Neritopsis ; they resemble the operculum of the 

 recent N. radiila. 



Several species have been described from U. Lias to Coral- 

 line Rag. P. bilobatum, Desl. Upper Lias of Normand3\ 



Family AMMONITID^. 



Animal contained in the last division of a chambered shell ; 

 protected bj^ one or two operculigerous plates (Aptychi); with- 

 out ink-bag. 



Shell external, of variable form, composed of two principal 

 layers, the inner one of which is nacreous ; sutural line of the 

 septa more or less complicated or lobed ; siphon simple, without 

 organic layer. 



Initial chamber ovoid, smooth, without exterior cicatrice, con- 

 taining a siphonal caecum free from the inner wall. Embrj^onic 

 shell generally showing an umbilicus at each extremity of its 

 axis ; first chamber convex in front. 



The above are the characters given to the order Ammonea by 



