THE CEPHALOPODA 139 



ill-defined tubercles along the peripheral margins, but on 

 the shell itself the periphery is marked by longitudinal 

 ridges and transverse striae. The septa are farther apart 

 than in Meloceras ; the siphuncle in the same position. 



8. Ophidioceras simplex (Fig. 45, b), from the 

 Silurian of Bohemia, is a flat-sided spiral shell, 35 mm. 

 in diameter, of several whorls which are just in contact, 

 except that a very small portion of the last whorl becomes 

 straight and so separates from the next inner whorl. The 

 aperture is constricted and Y-shaped, the hyponomic sinus 

 (the stem of the Y) being towards the periphery, so that 

 the shell is exogastric (as are most, if not all, of the spiral 

 cephalopods). The sides are ornamented by simple ribs, 

 interrupted by a keel on the periphery. 



These three examples illustrate stages in the transition 

 from a straight shell like Orthoceras to a tightly coiled 

 one like Nautilus. At one time these different degrees of 

 curvature were regarded as marking'different genera, and 

 generic names were given accordingly. Now it is recog- 

 nized that they are evolutionary stages which may have 

 been passed through by different stocks at the same or at 

 different times. Names for these stages are useful. 

 Following Mr. Buckman in modifying the more cumbrous 

 terms of Hyatt, we may call the straight shell an 

 orthocone ; the curved one which does not form a spiral. 

 cyrtocone ; the open spiral, a gyrocone ; the spiral with 

 whorl-contact, an ophiocone. The terminology does not 

 at present distinguish between evolute ophiocones like 

 Ophidioceras, in which the whorls merely touch, and those 

 in which, by tighter coiling, the inner whorls become 



