SUB-ORDER D ORTHOCHOANITES 521 
crests, some forms having altogether as many as five sinuses and five crests. Siphuncle 
tubular and usually large. 
Cyclolituites, Remelé; Lituites, Breyn (Fig. 1067); Angelinoceras, Holmiceras, 
Hyatt. Ordovician. Ancistroceras, Boll; Rhynchorthoceras, Remelé. Ordovician and 
Silurian. (For re-descriptions see Hyatt’s Phylogeny, 1894.) 
III. PLeEURONAUTILIDA. 
Comparatively smooth nautilicones, the primitive genera discoidal but leading up to 
some highly involute shells in the Trias. The later Mesozoic and Tertiary shells nearly 
all deeply involute. Some of the Triassic Clydonautilidae have more sinuous sutures and 
a greater number of lobes and saddles than any other Nautiloids, and this complexity 
persists, although to u lesser degree, among the Jurassic, Cretaceous, and Tertiary forms. 
Stphuncle tubular and small, with mostly short funnels except in Aturia, where they are 
very long. 
Family 9. Grypoceratidae. 
Primitive forms have discoidal volutions with very simple sutures, but are succeeded 
hy involute shells having more complex sutures. The latter have prominent ventral 
saddles sometimes divided by a lobe, and large lateral and dorsal lobes. All genera 
save one known to have annular lobes. Shells less highly ornamented than in preceding 
family, and sutures simpler than in the next following. 
Syringoceras, Hyatt. Discoidal with primitive, approximately tubular, or slightly 
compressed volutions. Surface marked by longitudinal ridges, sometimes intersecting 
the transverse lines so as to produce a cancellated surface. Sutures with faint ventral 
saddles, sight lateral and dorsal, and minute annular lobes. Siphuncle very small 
and near the venter. Trias. 
Grypoceras, Hyatt. Volutions more or less deeply involved, but umbilicus open, 
the venter narrow and often channeled. Sutures 
with narrow, sometimes deep ventral lobe, broad, 
sweeping lateral lobes, and deep dorsal with 
annular lobes. Siphuncle dorsad of centre, 
Trias. 
Family 10. Clydonautilidae. 
Shells have folds in some species, and all are 
deeply involute except the primitive genus Clymeno- 
nautilus, Lateral lobes of sutures more or less 
deep and often sub-angular, suggestive of the 
Clymenidae among Ammonoids, Some highly 
specialised and involute species have the wmbilical 
lobes exposed on the sides, and an additional pair 
of laterals developed near the venter, thus making 
three pairs of lobes on each side. The compressed 
volutions, narrow venter, and aspect of the young 
and primitive forms seem to indicate close affinity 
with the Grypoceratidae, but only a few species of 
late Mesozoic time are known to have annular lobes. 

Clymenonautilus, gen. nov. Smooth, dis- BAER e GS. 
coidal shells with more or less compressed Teen ce sp Sau 
volutions, and narrow convex venter. Sutures 
with prominent ventral saddles, one pair of deep lateral lobes, and large marginal saddles. 
Siphuncele supposed to be near the venter. Type C. (Nauwt.) Ehrlichi, Mojs. sp. Trias. 
Clydonautilus, Mojs. Deeply involved nautilicones with compressed volutions, 
