92 
THE VOYAGE OF H.M.S. CHALLENGER. 
Acera; and since these Bulloidea probably resembled Acera, we may still find a feature 
of resemblance in the “ proboscis ” of this latter and that of Peraclis and the young 
Cymbuliidse. 
I have said that the Thecosomatous Pteropods must have arisen towards the end of 
the Cretaceous or in the early part of the Tertiary epoch. Indeed, in the Secondary 
period there exists no Pteropod analogous to the Tertiary Thecosomata ; and, as I have 
already said, I cannot admit among the Thecosomata the so-called Primary “ Pteropoda.” 
There exists a considerable number of these fossils (more than a hundred species), 
which, not being assignable to any other group, have been placed among the Pteropoda 
on account of certain apparent resemblances. 
The absence, which has been already mentioned, of any organic remains in the 
Secondary rocks which could possibly be attributed to the Pteropoda, and the enormous 
interval of time which consequently separates these fossils from the true Tertiary Theco- 
somata, is of itself an argument against the interpretation which has been given by 
palaeontologists of these organisms. 
The only so-called “ Pteropods ” in the Secondary rocks are two species of Conularia 
analogous to those of the Primary formations— Conularia sp , Bittner, 1 from the Trias, 
and Conularia cancellata, Argeliez, from the Lias. 
In spite of the distance in time which separates the Primary “ Pteropoda ” from the 
true Thecosomata of the Tertiary period, the former have hitherto been always ranged 
among the latter, although only a small number of them show an external resemblance to 
certain species of Clio of the subgenera Creseis and Hyalocylix. 
The fossils which exhibit this supposed resemblance to the existing Cavoliniidae are 
as follows 
1. The “ Creseis ” and “ Styliola” of the Silurian and Devonian. These are fossils 
which are not very well preserved, have no embryonic shell like that of Clio, and often 
exhibiting a longitudinal striation such as is seen in no existing species of Clio. No real 
affinity can be found between these organisms and the genus Clio ( Creseis ) ; on the other 
hand, the great size of these Primary fossils separates them from all known forms of 
Pteropoda in the same manner as they are separated by stratigraphical considerations, for 
from the Devonian to the lower Tertiary there is no fossil which could be referred to an 
extinct Thecosome of this group. 
• 9 
As to the supposed specimens of Creseis of small size described by Ehrenberg, 2 their 
strong regular curvature, their oblique mouth, their apex without any distinct embryonic 
shell, separate them entirely from all the known Thecosomata, and render it impossible to 
unite them with the subgenus Creseis of Clio. 
1 Verhandl. k. k. geol. Beichsanst., 1878, p. 281. 
2 Ufcber massenhaft jetzt lebende oceanische und die fossile altesten Pteropoden der Urwelt, Monatsber . d. k. 
preuss. Ahad. d. TViss. Berlin, 1S61, tigs. 19-21. 
