THE CEPHALOPODA. 213 



The turns of the shell in contact, overlapping ; siphuncle near the outer (convex) 

 side ; edges of the partitions separating the chambers much waved, but not having at 

 their base small, convex, tooth-like projections ; from Silurian into Trias. 



51. Cltmenia, Miinster, 1832. =: Endosiphonites, Ansted, 1S3S. = Subclymenia, 



d'Orbigny, 1850. 



The turns of the shell in contact, overlapping; siphuncle near the inner (concave) 

 side ; edges of partitions waved with slight angular depressions. (This genus has 

 affinities with Aturia, but the turns of the shell are more overlapping in the latter, and 

 the siphuncle is larger). Devonian into Carboniferous. 



2. Shell straight. 



52. Bactrites, Sandberger, 1842. = Stenoceras, d'Orbigny, 1850. = Trematoceras, 



d'Eichwald, 1851. 



Siphuncle marginal ; septa gently waved, a single angular depression (lobe) ; external 

 chamber not known ; from Silurian into Devonian. 



X. — Shell many chambered ; the last (exterior) chamber large ; siphuncle marginal ; the 

 edges of the partitions (septa), which separate the chambers, niany-waved, with 

 single circular or subcircular depressions at their base. Eamily, Clydonitid^. 

 Four genera : Clydonites, Choristoceras, Rhabdoceras, and Cochloceras. 



1, Shell spiral, the turns touching, in the same plane. 

 53. Clydonites, Hauer, 1860. 

 Triassic and Cretaceous. 



2. Shell spiral, the turns not touching, in the same plane. 

 54. Choristocekas, Hauer, 1865. 



Triassic. 



Triassic. 



Triassic. 



3. Shell straight. 



55. Rhabdoceras, Hauer, 1860. 



4. Shell spiral, the turns touching, not in the same plane. 



56. Cochloceras, Hauer, 1860. 



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