1874.] 19 [Hyatt. 



descriptions and figures, we have the means of tracing both iEgoceras 

 and Psiloceras to a common stock. Therefore Quenstedt after all is 

 in the main correct, though the point of separation for the two 

 stocks, one the parent of the Arietidae, and the other of the Augula- 

 tidse, must be sought in the Trias and not in the Lias. The resem- 

 blances between the form and characteristics of the full-grown Amm. 

 incultum and the young of jEgoceras angulatum during the stage in 

 which the pilae stretch across the abdomen, and the channel is still 

 undeveloped, are numerous and convincing in this respect. 



.ffigoceras angulatum Waagen. 



Amm. angulatus Sch., Die Petref., p. 70. 



Amm. catenatus Sow., De la Beche Traite de Geol., p. 407, f. 67. 

 " " D'Orb., Ter. Jurass., Cepb., pi. 94. 



Amm. coluhratus 7i\&t., tab. 3, fig. 1. 



Amm. angulatus depressus Quen., Die Ceph., p. 75, pi. 4, fig. 2. 



Notwithstanding Oppel's reunion of this species with Charmassei 

 and Leigneletii of D'Orbigny, I cannot regard them as anything more 

 than closely allied species, since they differ in the young, as well as in 

 the adult and old age. The young appear to be smooth for about one 

 and a half whorls, then lateral tubercles appear. These spread upon 

 the sides into folds, which on the early part of the fourth, or last of the 

 third whorl, rapidly become true depressed pilae, and then begin to be 

 continued across the abdomen with a very decided forward bend in 

 the geniculae, and an acute angle on the abdomen. The furrowing 

 or lineal depression which obliterates the angle of intersection of the 

 pilae on the abdomen, is developed on the last half of the fourth 

 whorl. 



On the early part of the fourth whorl the shell has already the 

 abdominal lobe somewhat deeper than the superior laterals, and 

 these again very much deeper than the inferior laterals. The cells 

 broad and rather shallow, the superior laterals being a trifle shallower 

 than the inferior laterals, as in the Arietidae. 



On the first quarter of the fifth volution the bases of the superior 

 and inferior lateral cells and the tops of the superior lateral lobes, 

 have become trifid, or unequally divided, whilst those of the inferior 

 lateral lobes and auxiliary cells are equally divided. The abdominal 

 lobes are shorter than the superior laterals, though the cells maintain 

 their old proportions. 



In the full adult condition the characteristics of the septa differ 

 considerably from the Arietidae, but approximate to those of Psilo- 



