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Cranoceras. 



This genus was described in Genera of Fossil Cephalopods, 

 p. 281, for a series of cyrtoceran forms having in the Silurian rep- 

 resentatives like Cranoceras (Cyrt.) hospitale, sp. Barrande, PI. 

 "151;" nigrum, PL "127," and Turnus, PL "483" and "484." 



The whorls are subtrigonal with the dorsum, much wider than the 

 venter, which is apt to be elevated and subangulated. The young, 

 until they are quite large, are compressed elliptical in section, with 

 the ventro-dorsal diameter longer than the transverse, then expand- 

 ing more rapidly they become more depressed and take on the 

 subtrigonal outline, the dorsum broader than the venter, which in 

 some species changes subsequently into the nephritic with a slight 

 impressed zone, Fig. 43, PL viii. 



The sutures have ventral saddles, slight lateral lobes and slight 

 broad dorsal lobes, but in some species may be approximately 

 straight and in the young stages are of this character in most forms. 

 Considering the size of the shells the septa are remarkably close 

 and numerous, and only slightly concave. 



The siphuncle is propioventran and apt to be filled with radia- 

 ting deposits. The Silurian forms do not have the nephritic out- 

 line and also have no impressed zone at any stage, judging from the 

 large shell of Cranoceras twnus, which, although it has a nautilian- 

 like form in the large fragment described by Barrande, probably 

 did not coil very closely. 



The Devonian forms are, however, more interesting in connec- 

 tion with the history of the impressed zone. These can be included 

 under the names of Cranoceras (Cyrl.) depressum and Cranoceras 

 (Cyrt.) line a turn. 



In the Museum of Comparative Zoology, in the Schulze collec- 

 tion from Pelm near Gerolstein, in the Eifel, there is a specimen of 

 Cranoceras lineatum 159 mm. in length along the median lateral line, 

 transverse diameter of smaller end 45 mm., abdominodorsal 41 mm., 

 and diameters of larger end 109 mm. and 85 mm. This is evi- 

 dently a quick-growing and very large specimen, but showing no 

 signs of having been coiled. It has, however, near the larger end 

 on the incurved dorsal side a very faint impressed zone given in 

 the outline, Fig. 43, PL viii, traced from the specimen. Some 

 specimens do not exhibit this depression, but most of this species 

 do have similar depressions and some of these are so nearly straight 



