REPOET OX THE PTEROPODA. 
71 
of the large specimens, which, however, it is bound to do, if in the latter the contracted 
portion is absorbed and the posterior portion alone left. This residue ought obviously to 
be identical and superposable in individuals of any size whatever. 
The small-sized specimens, like the large, are individuals which will not increase 
further, which have attained then limit of growth, as is otherwise indicated by the 
complete development of the reproductive system. The smaller size of the shell depends 
on its surface being developed along a curve with smaller radius than in the large-sized 
individuals. 
On the other hand, there are several forms of Cavoliniidse, to which distinct specific 
titles are given, notably those which Boas calls “ Hyales plates,” where the union of the 
two lips of the shell by the so-called ‘ ' appareil de fermeture” has not been developed. 
All these forms, as we shall immediately show, are individuals which have not yet 
attained sexual maturity, and belong to species already known, as Cantraine first suspected. 
But this condition of immaturity, associated as it undoubtedly is with reduced 
development of the reproductive organs , 1 may be prolonged to a very late stage, and the 
shell may be very large before the formation of the “ appareil de fermeture.” This can 
be easily demonstrated by examining a large number of specimens, as for instance of 
Cavolinia tridentata at Naples. In this form, to which our attention was first directed 
by Dr. Paul Schiemenz, one finds, even at the same stage of development, considerable 
difference in size. 
It is certain that there are notable differences in the size of adult specimens (with 
completely developed reproductive organs, and with perfected closing apparatus) ; and the 
theory of the partial absorption of the shell must be dismissed. 
But as I have already pointed out, those young stages which we have discussed have 
been regarded as distinct species, and have been referred either to the genus Cavolinia 
( Hyalsea ) or to the genus Clio {Cleodora), or to a special genus, Pleuropus. 
And besides these entirely superfluous terms, we also find for the forms which 
properly belong to this genus a profuse superabundance of specific titles, just as in the 
cases of Clio and the Limacinidse. 
As these Thecosomata are pelagic animals with a very wide geographical distribution, 
there is no inconsiderable exhibition of variation in the form of the shell. Thus have 
arisen numerous variations, distinguished by very slight divergences. But on the basis 
of minimal distinctions, conchologists have not hesitated to establish a large number of 
“new” species. 
If we abstract the titles which ought to be referred to other genera altogether 2 
1 See Gegenbaur {Hyalsea complanata), Untersuchungen fiber Pteropoden und Heteropoden, pi. i. fig. 1 ; Souleyet 
{Hyalsea Isevigata), Voyage de la Bonite, Zoologie, pi. v. fig. 14; Huxley {Cleodora curvata), On the Morphology of the 
Cephalous Mollusca, Phil. Trans., 1853, pi. iv. figs. 4, 5. 
2 Also Nudibranchs, designated Cavolina (Bruguiere). 
