1876.] 373 [Hyatt. 



ference becomes unavoidable that the living chamber lias a tendency 

 to become like that of Gervilii. 



Starting from these Gervilii-like varieties o*f Brocchii, a series can 

 be followed which leads imperceptibly into Gervilii proper with its 

 coarse ribs even in the younger stages, and from thence into the 

 smooth, globular, and more involute forms of Steph. Brongniartii. 



The series from Gervilii to microstomum is not so complete, but I 

 think no one can examine the forms in Prof. Moesch's collection, the 

 Amm. Yrair of Oppel from the Parkinsoni-bed, without coming to 

 the conclusion that they show characteristics intermediate between 

 true Gervilii of the Humphriesianus-bed and the Steph. microstomum 

 of the Macrocephalus-bed. The form, size, ribs, and the fact, that 

 in many specimens microstomum, like Gervilii, does not become smooth 

 on the living chamber, except in old specimens, while in others the 

 form is much more altered and smoother at comparatively early age, 

 are all intermediate characteristics. Their meaning, however, was 

 not perceptible to me until I had become assured that true microsto- 

 mum had no lappets, and was found as the variety, Ymir, geologi- 

 cally lower than the typical form. 



The peculiarities of the larger Gervilii-like varieties of Broccldi are 

 exaggerated in the succeeding platystomum forms, in which the living 

 chamber presents the irregular form at a very early age, and is 

 usually smooth and much compressed laterally near the mouth. The 

 evidence appears to show that there is a line of forms leading from 

 the smaller Gervilii and Brongniartii through variety Ymir in the 

 Parkinsoni-bed to microstomum in the Macrocephalus-bed, and also a 

 line which connects the larger Brocchii through their Gervilii-like 

 forms, with the true stout-formed platystomum of the same bed. The 

 latter is more deficient than the former, since there are no intermediate 

 forms in the Parkinsoni-bed, but this is largely made up for by the 

 close resemblances of some of the adult forms, and of the young of 

 this species to the adults of the normal or untuberculated variety of 

 Brocchii. This view of the affinities also explains better than any 

 other the very close similarity of the stout form of the shells through- 

 out, and the peculiar aspect of the living chamber. 



Throughout the whole of these series we find similar phenomena to 

 those occurring in the series which spring from subcoronatum. Where- 

 ever growth is continuous throughout life, old age does not act very 

 distinctly upon the shell in the obsolescence of the ribs or decrease 

 in size of the whorl as a whorl, either in the individual or in the 



