Hyatt.] 888 [June 7, 



imating to Humphriesianum, from which it is sometimes difficult to 

 separate it. It is really a variety of Deslongchampsii, with more 

 elevated abdomen and narrower umbilicus. 



THIRD SERIES. 



Stephanoceras Blagdeni. 



Amm. Blagdeni Sow., Min. Conch., pi. 201. 



Amm. coronatus Zieten, Verst. Wurtt., pi. 1, fig. 1. 



Amm. Blagdeni D'Orb., Terr. Jurass., pi. 182. 



Amm. coronatus Quenst., Die Ceph., pi. 14, f. 1. 



This species, though attaining a large size with fewer whorls, has 

 a most remarkably close resemblance to the ancestral form, Cceloceras 

 Pettos, so close indeed that they are very similar, not only in the 

 form and characteristics of the adults, but in the sutures, and in the 

 general history of the development of the yonng. This greater sim- 

 ilarity is directly traceable to the very obvious fact that in this vari- 

 ety of the species the immature Pettos-Uke form, characteristics and 

 sutures, which are common also to the younger stages of all other 

 forms of this genus, are here more strictly retained throughout the 

 entire growth of the animal. This is so strictly carried out, that the 

 shell in most specimens manifests none of the old age characteristics 

 or retrograde metamorphoses previously described in other species, 

 i. e., in the decrease of the amount of involution and size of the 

 whorls. In other specimens great changes take place, but they are 

 very distinct from those of the purely Humphriesianum forms. They 

 are first manifested in the elevation of the abdomen, which becomes 

 rounder and more elevated during growth, and the adults become 

 similar to some forms of the next described species. The amount of 

 the involution does not decrease, nor the relative size of the whorl, 

 but the abdomen becomes more elevated and the sides rounder. 

 These forms are similar to nodosum in general appearance, but their 

 real affinity with coronatum alone stands the test of close analysis. 



Stephanoceras coronatum. 



Amm. coronatus Brug., Ency. Meth., p. 43. 



This species always has in the young, for periods of variable 

 length, according to the variety, whorls which closely resemble in 

 form and characteristics those of the adult of Blagdeni. 



