ZOOLOGICAL OBSERVATIONS IN THE SOUTH PACIFIC, 223 
the size of this young shell behind the line of division between 
it and the later formed shell-substance seemed to correspond 
roughly with the dimensions of the ovarian ovum, as stated by 
me in a former communication,! I at once became interested 
in Hedley’s suggestion. 
The term “nepionic ” was invented by Hyatt in 1890, and 
embodied by Jackson in his important memoir on the ‘ Phy- 
logeny of the Pelecypoda,’? and has been adopted by Pilsbry 
in the ‘Manual of Conchology.’ As I understand it, it is 
intended to be applied to the larval stages, sensu stricto, of 
all animals. To quote Hyatt, “ This term should be confined 
to the designation of stages of growth which are immediately 
continuous with later stages, and parallel or referable in their 
origin to the adults of allied existing or fossil forms which are 
not so remote as those from which the embryonic stages are 
derived.” 
The nepionic period is succeeded by the adolescent or neo- 
logic stages, which “‘ are of as great importance for tracing the 
genealogy of small groups as are the [nepionic] characters in 
larger groups.” 
In the case of those terrestrial Gastropods which lay large 
hard-shelled eggs full of yolk, the development and growth of 
the shell take place within the egg until the close of the 
nepionic period, by which time the shell has in many in- 
stances attained a very respectable size. Thus the nepionic 
shell of the above-mentioned molluscs is that portion of the 
true shell (as opposed to the embryonic shell) which develops 
within the egg. 
1 “Letters on Nautilus, &c.,” see ante, p. 175. 
* Jackson, Kopert Tracy, “Phylogeny of the Pelecypoda, the Avi- 
culidse, and their Allies,” ‘ Memoirs Boston Soc. Nat. Hist.,’ vol. iv, No. 8, 
1890. 
Proressor Hyarr introduced the term “nepionic ” in a foot-note to the 
preceding memoir as a substitute for the term “silphologic” suggested by 
him ina previous paper (Hyarr, ALpuEvs, ‘ Values in Classification of 
the Stages of Growth and Decline, with Propositions for a New Nomen- 
clature,” ‘Proc. Bost. Soc. Nat. Hist.,’ vol. xxiii, Part 3, 1887). 
1 am indebted to Mr, Hedley for these references. 
