GONIATITES. 73 



On PI. XIV, figs. 11 and li'. the /'(Ov-J.^v/vw stage shows at tlic fourth, fifth, 

 and sixth sutures. 



Neanic. — \\ hen tlie annnonoid in its growth no longer shows tlie characters 

 of its distinct ancestors, hut lias already taken on th(>sl^ of its own family, it may he 

 said to have left tiie larval stage proper and to have begun its youth. The ananeanic 

 is then the beginniisg of the adolescent period. G. erenlatrla at the seventh suture, 

 three-fourths of a whorl, and diameter of 0.8.5 nun., changes its foi-m mai'kedly; the 

 two pairs of lateral lohes become more pronounced, and tiie ventral lobe becomes 

 smaller in proportion; the coil leaves its close spiral and shows decided egression, 

 the umbilicus l>ecomes wider, while the cham])er )>(^conies actually narrower than in 

 ihe P((ro(h>< 'rn IS stage, as seen fi'om these figures: Width of ciianiber at diameter 

 0.74 nun. is U.T7 nun., at diameter 0.92 mm. it is O.O!* mm. The involution also 

 becomes less. At diameter 0.90 nun. and end of the first whorl a decided constric- 

 tion, marking a temporary mouth of the shell, makes its appearance. This stage 

 corresponds to the Upper Devonian and Carboniferous genus Prionoceras Hyatt," of 

 which /'. ilivii<uiii Muenster, of the Upper Devonian, and P. hi-li'dJiainnit de Koninck, 

 of the Lower Uarlioniferous, are tlie types. These species have broad, low, rather 

 evolute whorls, with wide umbilici and smooth surfaces, ornamented only with 

 periodic constrictions. The external sutures consist of an undivided, pointed, ventral 

 lobe, one pair of angular lateral lobes, and a second pair of rounded lo))es on the 

 umbilical border. The external saddles ai-e angular and the lateral saddles rounded 

 and broad. If the genus Prinnoci/ras is dropped, as now seems likely, it will be 

 necessarj- to give some other name to this stage of growth. The beginning of 

 the Prionoeeras stage is shown on PI. XIV, figs. 11 and 12, in the widening of the 

 umbilicus, egression of the spiral, and narrowing of the chamber. 



Dimensions of tlic specimen. 



iUllimeter. 



Diameter 0. 92 



Height of last whorl : 33 



Height of la^^t whorl from the jireceiling 23 



Widtii of last whorl 69 



Involution 10 



Width of umbilical ■ 30 



This stage begins at 0.86 mm. diameter, three-fourths of a whorl from the pro- 

 toconch, and lasts with little change except increase in size for two revolutions up to 

 a diameter of 2.26 mm., when the transition to GoniatifcK begins. 



Plate XV, figs. 1 and 2, shows a continuation of the Prionoceras stage, at If 



whorls. 



Dimensicms. 



Millimeters. 



Diameter 1. 29 



Height of last coil 45 



Height of last coil from the preceding 29 



Width of last coil 9.5 



Involution \'-> 



Width of umbilicus 50 



"Proc. Boston Soc. Nat. Hist., Vol. XXII, p. 328. 



