1874.] 31 [Hyatt. 



diameter, but still not so smooth or so advanced in senility as 

 D'Orbigny's figure of a much smaller individual. The young are 

 identical in their characteristics with the more flattened varieties of 

 Arnioceras miserabile, which have the pilse bent forward and of linear 

 aspect. 



Amm. maugenestii D'Orb. 



This species, as exhibited in Prof. Fraas' collection, shows a young 

 form which at 35 mm. in diameter is precisely similar to the adult of 

 bipunctatus at the diameter of 8 to 12 mm. with two rows of spines. 

 The abdomen is also elevated at this early age, though it afterwards 

 flattens slightly, and assumes the planicostan pilse in some specimens. 

 When 9 mm. in diameter, the specimen referred to above loses its 

 tubercles and begins to become narrower across the abdomen, and at 

 the diameter of 11 to 12 mm. the senile features exactly reproduce 

 the characteristics of the spineless or Masseanus form. 



In D'Orbigny's collection it may be seen that there is no real line 

 to be drawn between Maugenestii and bipunctatus (Valdani D'Orb). 

 The single row of spines described by D'Orbigny is confined to a 

 limited number of specimens; the larger number of specimens have 

 two rows of spines. The extreme Valdani or bipunctatus form is 

 usually thinner, and the septa somewhat different. 



Cycloceras Masseanum. 



That this is merely the old age of the preceding species, and not a 

 form by itself with adult characteristics, is shown by all the collec- 

 tions I have seen, especially by that of Dr. Schwartz, who first drew 

 my attention to this fact. 



Cycloceras iEgseon. 



This form is very closely connected to the preceding or Actaeon 

 variety of Maugenestii by a series of forms in which the old or Mas- 

 seanus stage is inherited at earlier and earlier periods, until it finally 

 invades the young stages of development. In other words, C. JEgaeon 

 is the old age type of its series, having no stage corresponding to the 

 adult, but only to the old age of the preceding species. 



Peronoceras acanthopsis. 



This is evidently only a variety of subarmatum, with a broad 

 abdomen. 



Peronoceras subarmatum. 



As pointed out by Quenstedt, this is identical with the flattened 

 forms named by him Amm. Bollensis, and occurring in Lias e. 



