94 
THE VOYAGE OF H.M.S. CHALLENGER. 
3. Cymbuliidge.- — The Silurian fossils described by Ehrenberg 1 under the name of 
Panderella, and regarded as larval shells of Cymbuliidse, 2 are coiled in a plane, and are 
bilaterally symmetrical like those of Bellerophon or Oxygyrus. 
As to the fossils referred with a “ ? ” to larval shells of Cymbulia 3 and Tiedemannia 
( = Gleba ), 4 they are entirely uncoiled, the turns of the spire not being in contact ; and 
in most cases they are coiled in one plane, neither spire nor umbilicus being visible. 
There is thus no connection between these fossils and the larval shells of Cymbuliidse. 
To sum up, we see that in the case of all these Primary so-called “ Pteropoda ” there 
is no reason whatever to regard them as Thecosomata. One palseontologist even has 
recognised the improbability of the organisms being referable to the Pteropoda : Hoernes, 5 
in speaking of Conularia and Hyolitlies, says that they “ perhaps form a group distinct 
from the Pteropods and of unknown affinities.” 
I am strongly inclined to believe that among these Primary “ Pteropods ” there are 
organisms belonging to different groups, but I am unable to decide which ; and perhaps, 
even after a prolonged study, it would be impossible to class them with any known 
living organisms. What I can definitely assert, however (and Boas, whose authority on 
this point cannot be doubted, has arrived at the same opinion 6 ), is that not one of them 
has the least affinity of any kind whatever with the Pteropoda, and that these latter are 
only to be discerned with certainty at the beginning of the Tertiary period. 
B. Origin of the Gymnosomata. 
We have already shown that the Gymnosomata are closely related to the Aplysioidea. 
Just as we consider that the Thecosomata are descended from the Bulloidea, so we are 
persuaded that the Gymnosomata have arisen from Aplysioid ancestors, and we have 
already expressed this opinion several times. 7 
In the present instance we cannot, as with Thecosomata, call palaeontology to witness. 
The shell of the Aplysioidea is quite rudimentary, scarcely calcified, and but little adapted 
to fossilisation ; and in the Gymnosomata both mantle and shell have entirely disappeared 
in the adult. 
In Notarchus among the Aplysioidea, the mantle is already extremely reduced, and 
the shell has become microscopic, being lodged a little behind the anus. 8 Thus this form 
1 Leber mnssenhaft jetzt lebende oceanische und die fossile altesten Ptcropoden der Lrwelt, Monatsber. d. k. preuss. 
Akad. d. Wiss. Berlin, 1861, p. 434. L T eber die Obersilurischen und Devonischen microscopischen Pteropoden, 
Polythalamien und Crinoiden bei Petersburg in Russland, Ibid., 1862, pp. 599, 600. 
2 Ueber massenbaft jetzt lebende, &c., loc. cit., figs. 1-9. 3 Ibid., figs. 10, 11. 
4 Ibid., figs. 12-18. 6 Manuel de Paleontologie, p. 373. 
6 Spolia atlantica, pp. 94, 95. 
7 Description d’un nouveau genre de Pteropode Gymnosome, Bull. Scient. Dip. Nord, 1886, p. 226 ; and Zool. 
Chall. Exp., part lviii. p. 67. 
8 Vayssiere, Recherches zoologiques et anatomiques sur les Mollusques Opistobranches du Golfe de Marseille, 
L Tectibranches, loc. cit., p. iii. fig. 81. 
