X INTRODUCTION. 



are, as nearly as may be, those of Professor Alpheus Hyatt, ^ 

 as amended by Messrs. S. S. Euckman and F. A. Uather.'' The 

 terminology alluded to is as follows : — 



1. Embryonic. "This stage includes all individual history from 

 the ovum up to the time when the organism can be referred 

 definitely to its class." In the fossil state we can only have left 

 for investigation the hard part of the organism, which in the case 

 of the Gasteropoda is here called the Protoconch, and in the 

 Lamellibranchiata the Pyo(iissoeo7i'ch. 



2. Brephic. This stage immediately succeeds the Embryonic, 

 and it is generally undei-stood that during it no specific characters 

 can be distinguished ; but this point will be discussed presently. 



3. Neanic. "Luring this stage specific characters and all other 

 morphological features present in the adult, appear and undergo 

 development." 



4. Eplielic. That stage denoting "the period of full develop- 

 ment of the individual, when all specific characters are clearly 

 recognizable." 



5. Gerontie. "During this stage changes take place which are 

 due to gradual failure of powers." It is subdivided into : " a, 

 Catahatic stage, in which the individual loses its ephebic characters ; 

 and h, Hypodrophic stage, in which the continued loss of characters 

 causes pronounced reversion." 



In view of the fact that so little is known concerning the 

 phylogeny of the Gasteropoda, I have found it somewhat difficult 

 in certain cases to apply this terminology satisfactorily. Por 

 instance, in the case of shells that are quite smooth it is hard 

 to say where the protoconch ends and the brephic stage begins ; 

 but in very many instances a characteristic varix is present, 

 denoting the termination of the former. The brephic stage, as 

 a rule, in reference to these fossils, was a short one, the growth 

 having been very vigorous immediately on the animal's escape from 

 the capsule, and conchological specific characters consequently 

 make their appearance at a very early stage. 



Some of the more prominent results obtained from this study 

 of Australasian shells may now be refisrred to. 



Proc. Boston Soc. Nat. Hist. vol. xxiii. 1888, p. 396. 

 Zoologiscliea Auzeiger, Nos. 405-6, 1892, pp. 420, 429. 



