602 



MOLLUSCA 



PHYLÜM VI 



II I. Pleuronautilida Hyatt. 



Gomparatively smooth naufAlicones, the primitive genera discoidal hut leading up to 

 some highly involute shells in the Trias. The later Mesozoic and Tertiary shells nearly 

 all deeply involute. Some of the Triassic Glydonautilidae have more sinuous sutures 

 and a greater number of lobes and saddles than any other Nautiloids, and this 

 complexity persists, although to a lesser degree, among the Jurassic, Gretaceous and 

 Tertiary forms. Siphuncle tubulär and small, with mostly short funnels except in 

 Äturia, where they are very long. 



Family 9. Grypoceratidae Hyatt. 



Primitive forms have discoidal volutions ivith very simple sutures^ but are succeeded 

 hy involute shells having more complete sutures. The latter have promAnent 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 tubulär, or sliglitly 

 compressed volutions. Surface marked by lougitudinal ridges, sometimes intersecting 

 the transverse lines so as to produce a cancellated surface. Sutures with faint ventral 

 saddles, slight 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 channelled. Sutures with narrow, sometimes deep ventral 

 lobe, broad, sweeping lateral lobes, and deep dorsal with annular lobes. Siphuncle 

 dorsad of centre. Trias. 



Family 10. Glydonautilidae Hyatt. 



Shells have folds in some species, and all are deeply involute except the primitive genus 



Glymenonautilus. Lateral lobes of sutures more 

 or less deep and often sub-angular, suggestive of 

 the Glymenidae among Ammonoids. Some highly 

 specialised and involute species have the umbilical 

 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 in- 

 dicate dose affinity with the Grypoceratidae, but 

 only a few species of late Mesozoic time are 

 known to have annular lobes. 



Glymenonautilus Hyatt. Smooth, discoidal 

 Shells with more or less compressed volutions, 

 and narrow convex venter. Sutures with pro- 

 minent ventral saddles, one pair of deep lateral 

 lobes, and large marginal saddles. Siphuncle 

 supposed to be near the venter. Type G. 

 {Nautilus) ehrlichi (Mojs.). Trias. 

 Fin. 1117. Glydonautilus Mojs. Deeply involved 



Herc<^lossaf^n«n^a(0^^)^j„^rj.ra ; „autilicones with compressed volutions, narrow 



concave venter, and umbilicus small or closed. 

 Sutures with prominent ventral saddles undivided by ventral lobes in adults hells. Trias 



