1876.] 367 [Hyatt. 



early as 1846 he adopted the mode of work which is fast becoming 

 universal, that of uniting in the same genetic series all forms, however 

 dissimilar in aspect, which can be traced into each other, by means 

 of the young and of the adult characteristics. 



Stephanoceras 1 Waagen (Pars). 



The earliest observed form of this genus is the Stepli. nodosum = 

 Humjphriesianus nodosus Quenst., which occurs in the Humphriesianus- 

 bed. This variety or species, whichever the taste of the reader 

 prefers, has the ribs more prominent and more widely separated than 

 in Humphriesianum, the umbilicus larger, and the whorls increasing 

 more slowly in size by growth ; this renders the shell altogether more 

 discoidal in aspect. The varieties, however, show a shading of the 

 characteristics in three different directions. One way leads to Steph. 

 Bayleanum, and another to Stepli. Humphriesianum, and still another 

 to subcoronatum. Towards Bayleanum a retrograde series of changes 

 produces forms more and more discoidal, with whorls increasing more 

 and more slowly in size by growth, until in the typical Bayleanum a 

 very distinct species appears, as figured by D'Orbigny, and discussed 

 by Oppel. It occurs contemporaneously with nodosum, and also later 

 in the upper part of the Humphriesianus-bed. 



In a similar way, by following the indications of the gradually 

 changing varieties we are led to the stoutest, most involute and 

 narrow-umbilicated forms of the typical Steph. Humphriesianum. In 

 these the abdomen is also more elevated and rounder, the ribs are finer 

 iand more numerous, and the sutures distinct. Steph. subcoronatum, 

 as pointed out by Oppel and Quenstedt, is one of the transition forms 

 of Humphriesianum, but it has a wider significance when carefully 

 studied in all its varieties. It becomes identical with A mm. Deslong- 

 champsii when the ribs are curved and prominently tuberculated, 

 and the abdomen somewhat elevated, though still very broad. The 

 abdomen becomes in some specimens still more elevated, the umbili- 

 cus narrower than in the Deslong champ sii, the umbilical shoulder of 

 the whorl more abrupt, the umbilicus deeper, the abdominal ribs par- 



1 This name, as lias been pointed out to me by E. B. Tawney, Esq., of Bristol, 

 has been already occupied by Ehrenberg for a genus of Rotatoria, but the termina- 

 tion adopted was spelled with an " o " instead of an " a," Stephanoceros instead of 

 Stephanoceras, and this seems to me quite sufficient under the circumstances to 

 justify its retention. 



