60 AMMONITIDA. 
situated within the centre, or between the middle and the inner 
margin, and not funnel-shaped, but tubular and gradually 
tapering. Distr.—Kocene, 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, 1876. 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. 
CRYPTOCERAS, d’Orb., 1847. (Solenochilus, Meek and Worthen, 
1877.) Planorbiform; septa arcuated, without lobes or sinuos- 
ities; siphon dorsal. Distr.—2 sp. Devonian, Carboniferous ; 
Europe. OC, subtuberculatus, V’Orb. (xxxi, 504). 
‘ 
BeEAKS OF TETRABRANCHIATES. 
These are found associated with fossil Nautiliand occasionally 
Belemnites, but never with Ammonites. The upper beaks have 
been described under the name of Rhyncholites, the lower ones 
as Conchorhynchus. 
R. Astieriana, d’Orb. (xxxv, 74). 
C. avirostris, Bronn (xxxv, 75). 
C. Owenii, Bronn (xxxv, 76). 
PELTARION, Deslongchamps. This was formerly 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 Peltariz are opercula of 
fossil species of Neritopsis; they resemble the operculum of the 
recent N. radula. 
Several species have been described from U. Lias to Coral- 
line Rag. PP. bilobatum, Desl. Upper Lias of Normandy. 
Famity AMMONITID#. 
Animal contained in the last division of a chambered shell ; 
protected by one or two operculigerous plates (Aptychi); with- 
out ink-bag. 
Shell external, of variable form, composed of two aera 
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 cecum free from the inner wall. Embryonic 
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 
