464 MR. S. S. BTJCKMAN ON THE GROUPLN'G OF SOME [Aug. 1 898, 



Genus C(eloceeas, Hyatt. 



It seems to me desirable to take Ammonites pettus, Quenst., as the 

 type of this genus, and to restrict its use to those species showing 

 the similar characters of a broad periphery, nearly flat from spine 

 to spine, of depressed whorls, and crater-like umbilicus : that is, if, 

 as would appear to be the case, these characters are due to direct 

 descent, and are not independent developments in different series. 

 These characters attain their maximum development in Blagdeni, 

 Sow., and the same form of shell is found to characterize the young 

 of the ^roccAz-group, of the Deslongcliampsi-gvoVi^^ of the Hum- 

 phriesiaiium-gTOu^, and even the infant stages of ParJcinsonia and 

 its allies. These facts seem to indicate that Cceloceras is the radical 

 form from which these others are offshoots. 



The principal species of the genus as now constituted would be 

 Coeloceras pettus (Quenst.), C. Blagdeni (Sow.), G. BanJcsi (Sow.), 

 and C. coronatum, Brug. (d'Orb.). Ammonites centaurus, d'Orb., 

 of the Deroceratan stage, is also related ; but exactly in what manner 



is not easy to say at present. 



Genus Stepheoceeas, S. Buckm. 



This is only an alteration of the name Stephanoceras^ because 

 that was preoccupied when proposed by Waagen. The type- 

 species, however, remains the same — namely, Stepheoceras Hum- 

 phriesianum (Sow.). The distinction from Coeloceras is principally 

 one of development — namely, that the broad flat periphery gives 

 place at an early age to a rounded and narrower periphery. The 

 broad flat periphery only yields to the arched periphery in Coelo- 

 ceras in the gerontic stage (see C. BanJcsi) ; but in StepJieoceras 

 the change takes place in immaturity. Thus what is only a 

 gerontic and occasional feature in Coeloceras becomes the ephebic 

 and regular character of Steplieoceras. Almost accompanying 

 this feature is another — the recession of the inner margin from 

 the line of tubercles, whereby in old Coeloceras, and in certain 

 Stepheocerata, a so-called ' abnormal ' body-chamber is produced/ 

 but as groT\i;h proceeds on the same lines, and in other species the 

 change takes place at an earlier age, what is a so-called ' abnormal 

 body-chamber* at the time of the change becomes a perfectly 

 normal feature, with almost imperceptible recession. 



Thus there are really three styles of umbilication in Stepheoceras : — 

 1st, in the immature whorls, up to a time which varies with the 

 species, regular concentric umbilication, with the inner margin of 

 the overlapping whorl in contact with the spines of its predecessor ; 

 2nd, a period when the umbilication becomes excentric, owing to 

 recession of the inner margin from the line of spines ; 3rd, a period 

 of renewed concentric umbilication, when practically the whorls 

 coil concentrically, so far as each is concerned with the other, but 

 all of them coil at a distance from the line of spines, so that the 



^ See d'Orbigny, ' Cepb. Teri\ Jurass.' pi. cxxsiv, Ammonites HumpJiriesiantis. 



