68 



CEPHALOPODA. 



coiled into a flat spiral, and closely resembling some of the 

 older forms of Nautilus. The inner side of each whorl is 

 deeply excavated for the reception of the convexity of the 

 internal whorl. The septa are simple, like those of Nautilus, 



or are slightly lobed, and 

 the siphuncle is inter- 

 nal, placed on the con- 

 cave side of the whorls. 

 Numerous species of 

 Clymcnia are known, 

 all the typical forms 

 belonging to the De- 

 vonian period ; and 

 some of the Upper De- 

 vonian limestones of 

 Germany are so pro- 

 fusely charged with 

 fossils of this genus 

 as to have received 

 the name of " Cly- 

 menienkalk." 



From the general 

 lol)ation of its septa, 

 Clymcnia is clearly to 

 be approximated to Goniatitcs, with which the genus is 

 sometimes placed. The forms with simple septa occur 

 both in the Silurian and Devonian, and are probably best 

 regarded as a distinct genus — namely, Trocholitcs. Aturia, 

 again, of the Tertiary period, has thick walls and a large 

 siphuncle, while the septa have a large lateral lobe. Lastly, 

 Gyroceras, principally Devonian in its range, though also 

 occurring in the Silurian, may be placed here. The shell 

 is in the form of a flat spiral, but the volutions are not in 

 contact, the siphuncle is placed on the convex side of the 

 shell, and the septa are simple. This genus, therefore, 

 resembles the Ammonitidm in the position of the siphuncle, 

 but agrees with the Nautilidm in the possession of simple 

 septa. 



Sub-family 2. Orthoceratid.e. — Shell straight, curved or 



Fig. 454, — Cbjmenia Sedgwickii. Devonian. Tlie 

 lower figure shows tlie form of the suture. 



