﻿528 



and the angle of growth so convergent that it becomes difficult, 

 perhaps impossible, to attribute the existence of this zone to con- 

 tact and pressure of a coiled whorl, unless it was acquired by inher- 

 itance through some unknown closely coiled forms. 



None of these specimens have the double impressed zone figured 

 in Cranoceras ( Cyrtoceras) depresswn, by D' Archiac et De Verneuil,* 

 but I have studied some fragments of this species showing the same 

 peculiarity. The two latero-dorsal impressions or faces and the 

 central gibbous dorsal face give an outline similar to that of the 

 young of the Trocholites'canadense, given in Fig. 24, PI. iv, of this 

 paper. The history of the appearance of this modification in this 

 large adult whorl, arising as it does from the direct modification 

 of the younger rounded dorsumf without being preceded by the for- 

 mation of an impressed zone is, however, entirely distinct from 

 that which occurs in the paranepionic substage of Trocholites. In 

 several genera of Carboniferous nautiloids (ex. Asymptoceras, 

 Apheleceras) similar faces appear on the dorsum, but the central, 

 gibbous dorsal face is fitted into the hollow flute or ventral zone of 

 the next inner whorl and is obviously a result of close-coiling and 

 adaptation of the plastic dorsum of the growing external volutions 

 to the ventral modifications of the inner volution. 



In Solenocheilus of the Carboniferous, however, the whorl has a 

 rounded venter and yet notwithstanding this a gibbous dorsal face 

 and dorso-lateral concave faces or furrows are formed independently. 

 In Cranoceras depression the origin of the gibbous dorsal face and 

 latero-dorsal faces or furrows appears also, so far as the facts go, to 

 have been entirely independent of any correlation with the ventral 

 surface, which is rounded and gibbous. These characteristics do 

 not seem to have had a mechanical origin in any of the shells, so 

 far examined, which have the dorsal side free or comparatively free 

 from contact. 



A very large and remarkable specimen in the Schulze collection, 

 Mus. of Comp. Zoology, shows a very short living chamber, which 

 has an aperture very broad transversely and with a nephritic out- 

 line and apparently very broad and well-marked impressed zone. 

 This species is not a variety of lineatum, but a distinct species pre- 

 cisely similar to D'Archiae and De Verneuil's figures of Phragmoce- 

 ras subventricosum, but the siphuncle is ventral. 



* Geol. Trans. London, 2d ser., vi, PI. xxix. 



fThis is also figured by Roemer, Hdrzgeb. Paleontogr., iii, PI. vi, in a young specimen. 



