212 
NAUTILOIDEA. 
vature, to one in which a complete whorl is attained. The curved 
forms are those which were formerly set apart to constitute the 
genus PJiragmoceras. In the straighter forms the ventral side, or 
that upon which the siphuncle is situated (corresponding with the 
smaller apertural orifice), is always more convex than the dorsal. 
In the curved forms, on the other hand, the dorsal side (correspond- 
ing with the larger apertural orifice) is convex, the curvature in 
fact being reversed in the two cases, as may he seen in the diagrams 
a, 5, fig. 34. 
a. Exogastric type 
( = Gomphoceras, as 
formerly restricted). 
Fig. 34. 
h. Endogastric tj’pe 
(^=Phragmoceras, as 
formerly restricted). 
Siphuncle. 
For these two types the terms “ exogastric ” and “ endogastric,” 
proposed hy Saemann ^ will be useful. The former implies that the 
siphuncle lies near the convex side of the shell, and the latter that 
it is near the concave side. The straighter (exogastric) forms vary 
considerably in their outline, according as they are more or less in- 
flated ; so that they may he pear-shaped, amphora-shaped, or barrel- 
shaped, e. as viewed ventrally or dorsally, because the lateral 
outline is modified by the greater convexity of the one side in com- 
parison with the other. 
In the more curved (endogastric) forms the increase in diameter 
is more rapid, though there is always a slight diminution towards 
the aperture. The shell commences with a pointed, more or less 
attenuated apex, and the initial point is described by Barrande 
Palaeontographica, 1854, Band iii. p. 139 ; also Barrande, loo. cit p. 262. 
