MOLLUSCA. CEPHALOPODA 265 



occupies most of the outer side ; the septa join together and then 

 bend round and divide again before reaching the inner side of the 

 shell ; the siphuncle of this part is very short. Ordovician, but 

 chiefly Silurian. Ex. A. bohemicum, Silurian. 



Cyrtoceras. Similar to Orthoceras but always curved : never 

 forming a complete volution ; section usually elliptical or oval. 

 Siphuncle small, cylindrical or bead-like, usually sub-marginal and 

 usually near the convex side of the shell. Cambrian to Carboni- 

 ferous. Ex. C. lineatum, Devonian. 



Poterioceras. Shell fusiform, slightly curved, inflated in the 

 middle, contracted at both ends, but especially at the apical end. 

 Section elliptical in the adult. Siphuncle sub-central or marginal, 

 inflated between the septa. Last chamber large, aperture simple, 

 contracted. Ordovician to Carboniferous. Ex. P. fusiforme, Car- 

 boniferous Limestone. 



Nautilus (fig. 103). Shell more or less globose, spiral, whorls 

 few, coiled in one plane, and more or less completely embracing. 

 Umbilicus usually small or absent. Last chamber much larger 

 than the preceding one, aperture simple, with an external sinus. 

 Septa concave, sutures more or less undulating. Siphuncle central 

 or sub-central, septal necks short and directed backwards. Surface 

 of shell smooth or ornamented with strise or ribs. Trias to present 

 day. Ex. J\ T . po/iipilius, Recent ; N. pseudolineatus, Inferior Oolite. 



Discites ( = Discitoceras). Shell spiral, compressed, discoidal ; 

 whorls quadrangular in section, increasing in size gradually, some- 

 times a little embracing, with the external margin flat or grooved, 

 and the sides flattened. The last part of the shell is separated 

 from the preceding whorl for a short distance. The earlier 

 whorls with longitudinal ribs. Carboniferous Limestone. Ex. D. 

 leveilleanus. Other Carboniferous genera with plane spiral shells 

 are Coelonautilus, Temnocheilus, Vestinautilus. 



Aturia. Shell discoidal, whorls compressed, completely em- 

 bracing, with rounded external margin ; suture-line zigzag, with a 

 deep angular lobe on each side. Siphuncle near the internal 

 margin ; septal necks large and very long, completely covering the 

 siphuncle. Eocene and Miocene. Ex. A. zic-zac, Loudon Clay. 



