30 Guide to the Invertehrata. 



Nautilidae. Pleuronautilus has strong transverse costae ; it occurs in the 



GALLERY Devonian, and extends into the Trias. Temnocheilus, ranging from 

 VII . . ' o o 



the Devonian to the Carhoniferous, has a broad periphery and the 



71, prominent lateral portion of the whorl usually angular, and 



sometimes ornamented with obtuse tubercles. 



In the Carboniferous genus Solenocheilus (Mg. 53), the shell 



generally has the form and surface characters of Nautilus proper, 



the siphuncle is near the periphery, and the lip on each side is 



drawn out into a narrow spout-like projection. 



Fig. 53. — Solenocheilus conspicuum, De Koninck. Carboniferous Limestone: 

 Ireland. (Reduced.) 



Table-case Commencing in the Trias, the genus Nautilus is numerously 

 cases 2 & represented in the Jurassic, Cretaceous, and Tertiary rocks, and 

 13. about two species are found living at the present day. 



{h) Ammonoiuea. — Turning from the Nautiloidea to the Ammo- 

 Table-case noidea, we have such forms as Bactrites, Clymenia, and the large group 

 of the Goniatites. In Bactrites the shell is small and straight, the 

 siphuncle marginal, and the sutures simple like those of Orthoceras. 

 In Clymenia the shell is coiled into a flat spiral, the whorls of 

 which are in contact, the siphuncle is large and on the inner 

 side of the shell, and the suture-line is simply folded or lobed. 

 Both genera are found in the Devonian, the former also occurs in 

 the Silurian. 



