Hyatt.] 396 [June 7, 



tion shows that it is after all not such as to indicate a genetic con- 

 nection between them. The angularity of the abdomen of Amm. 

 Goliatlius is wanting, and the flat abdomen of the earlier stages in 

 that group. The whole development is similar to that of Brocchii, 

 and it is only a stouter form of Gervilli, with a tendency to form a 

 smooth living chamber. 



The living chamber is evidently almost entirely absorbed during 

 the growth of the shell, as may be seen in all large collections. In 

 some specimens of considerable size the living chamber is smooth 

 only for a very short space near the mouth; in others of the same or 

 even smaller dimensions, nearly the whole is smooth. In very large 

 specimens, however, the living chamber appears to be invariably 

 smooth. The irregularity of the growth begins invariably in all 

 specimens near the base of this chamber by the contraction of the 

 whorl, and continues throughout. The increase in size, however, is 

 regular at all preceding periods, whatever the size of the shell. The 

 conclusion is therefore unavoidable, that the living chamber must be 

 almost wholly absorbed in the course of growth. The young are 

 precisely similar to the adults of Brocchii. 



NINTH SERIES. 



Stephanoceras dimorphum. 



Amm. dimorphus D'Orb., Terr. Jurass., pi. 141. 



The young of this remarkable species at first sight appear to be 

 identical with those of Brongniartii or Gervilii, but the permanent 

 mouth furrows marking the shell even at an early period, show it to 

 be distinct in its mode of growth. These appear to indicate that 

 the growth of the shell is constant, and that the walls of the living 

 chamber are never absorbed. If so, we have a very remarkable 

 change in the mode of growth. The young evidently retain the 

 Brocchiian living chamber until a late period of growth. That is, 

 the living chamber did not exhibit contraction in the young, but like 

 that of Brocchii, continued to increase in size towards the mouth 

 except in old specimens. As the specimen reached the adult condi- 

 tion, however, in this species the chamber assumed the usual propor- 

 tion of that part in Brongniartii, and continued to decrease until the 

 death of the animal. This appears to be the only way in which to 

 account for the presence of the permanent mouth furrows. 



Comparisons of the young with those of Gervilii and Brongniartii, 

 seem to indicate a very close affinity; but this evidence, and the 



