1873.] 167 [Hyatt. 



is the species known as planorbls in England, and psilonotus in Ger- 

 many. 



In tracing the different series of species, it has been found that the 

 forms differ in their adult characteristics, sometimes very decidedly, 

 and in other instances hardly any definite line can be drawn between 

 those of the same series. Thus Arnioceras, comprehending forms as 

 distinct as the miserab'dis of Quenstedt, and the Ceras of Giebel, may 

 be described as one species; again torus, tortilis, Liassicus and Nodo~ 

 tianus are all distinct forms, and yet undoubtedly derivatives of the 

 common stock, torus. It would be entirely ridiculous to describe the 

 latter series as one species, but the former can be for the most part 

 included under the name of Amm. falcaries Quenstedt. The differ- 

 ence is explained by the study of development. 



Leaving out the first, or egg stage, common to all Ammonoid 

 forms, and the Goniatitic, or second stage, common to all the Am- 

 monites proper, the subsequent, or smooth stage, in Arnioceras mis- 

 erab'dis and its derivatives, occupies a notable proportion of the 

 umbilical whorl, and has a peculiar form resembling that of the adult 

 planorbis. The adult characteristics are slowly and methodically 

 added after this stage of growth. Some individuals barely attain the 

 form common to the genus; others, however, reach beyond this, and 

 add by growth channels, and otherwise modify the form. In all 

 cases, however, no sudden changes are made by the growth; each 

 adult passes through a normal course of development, and the differ- 

 ent characteristics distinguishing this or that variety or species, is 

 gradually acquired, no abrupt changes being remarked. 



In Caloceras, however, while all the young forms resemble torus, 

 there is great latitude in the assumption of the adult form and char- 

 acteristics, and differences, even in forms which must be regarded as 

 varieties of the same species, are introduced suddenly during the later 

 stages of growth. 



The study of the adult individuals in Arnioceras would enable an 

 observer to unite them, perhaps even into one species, as Quenstedt 

 and others have done, an 1 the study of the young would lead to a 

 parallel result; but in Caloceras, while the consideration of the adults 

 alone would lead to the distinction of numerous species, the investi- 

 gation of the development alone would indicate here, as in Arnioce- 

 ras, only one species. 



Throughout the Ammonoids, Ave find everywhere instances of these 

 two methods, the slow accumulation of differences, according to the 



