10 GENESIS OF THE ARIETIDAL. 
chamber perhaps longer in proportion. The entire absence of a cecum, and of 
all signs of a siphon, may be inferred with probable certainty in this first stage ; 
and we proposed, in the paper referred to above, to name it the Asiphonula. This 
form may indicate the previous existence of a common univalve ancestor for the 
Cephalopoda which resembled the Pteropoda. Certainly the aspect of the calcare- 
ous protoconch of Ammonoids and Belemnoids favors this idea, first suggested by 
Von Jhering ; and the asiphonula adds another argument, since it has no siphon or 
true septum. The young of the 
Pteropoda, especially the ancient 
forms, had calcareous protoconchs 
in most forms; but doubtless there 
TBR, “AW are more primitive shells in which 
iy ff D> \ the protoconchs had _ the more 
// primitive, embryonic, . conchioli- 
nous stage of development. The 
evidence, therefore, is not conclu- 
sive, but it justifies the supposition 
that Cephalopods and Pteropods 
had originally some common an- 
cestor, a true shell without septa 
or siphon, and possessing a proto- 
conch, which might have been 
conchiolinous. 
There is, however, another 
group, the Scaphopoda, which 
may claim to be considered in 
this connection. According to 
W. K. Brooks, the veliger is rep- 
resented by the adult of Dentalium 
Fig. 
1-8. Apex and protoconch of Orth. elegans, Miinst., from 
the side, below, and in front. In Fig. 2 the fine stria really 
cross the shoulder of the apex (b), and reach to the proto- 
conch (a). Named by Klipstein, Loc. St. Cassian, Coll. Brit- 
ish Museum. 
. 4,5. Apex and protoconch of another specimen mounted 
with the first on the same card. Named by same, Loc. same, 
Coll. same. 
ig. 6-8. Views from the side, front, and below of the same 
parts in Orth. politum, Klipst. The shading on the protoconch 
of Fig. 8 does not indicate structure; this protuberance is 
smooth. a, protoconch; b, shoulder of the area of the cica- 
trix. Named by same, Loe. same, Coll. same. 
in several of its leading charac- 
teristics, and this must be regarded 
therefore as the most generalized 
type of the true Mollusca. It is 
quite possible that the asiphonula 
may have retained some of the 
characters of the veliger, and may 
have resembled Dentalium or 
some common ancestor, and may have descended from this form without having 
passed through any pteropod-like ancestral modification. The peculiar resem- 
blances of the young of some of the Goniatitine and the adults of Tentaculites 
among Pteropoda may be entirely due to homoplasy, and not to homogeny.’ 
phenomena with w 
independent origin of similar ch 
ous use of terms Heterology and Homolo 
of Genera, 
1 These terms were first used by Lankester (Jour. Micr. Sci., XVII., 1877, p. 486). They express 
hich naturalists have long been familiar, ‘‘ homoplastic ’? meaning representation and 
aracters, and ‘* homogenous’? meaning genetic connection. See also previ- 
gy for the same phenomena by Cope, in his masterly essay, ‘‘ Origin 
” Proc, Acad, Sci. Phila., 1868, and “ Origin of the Fittest,’ p. 90. 
