EEPOET ON THE PTEEOPODA. 
9 
In their entire organisation (existence of a pallial cavity ; number of tentacles ; position 
of the penis, fins, cerebral ganglia, &c.) they agree with the Thecosomata, as we shall see 
in our anatomical Eeport. On the other hand, their special characters make it convenient 
to erect for them a special family, which appears to be a far preferable course to uniting 
them, as Woodward 1 has done, with the Cavoliniidse, in which he has also included the 
Gymnosomatous Halopsyche. 
As to the other Thecosomata, they form a more uniform group, within which one 
may pass from one form to another without remarking any very considerable modification. 
It is true, however, that in this group some forms differ from the majority in having the 
shell twisted into a spiral, as also in the position of the anus and of the pallial cavity. 
The existence of these last two differences in forms otherwise closely allied will be 
explained in the anatomical part of the Eeport. The differences just mentioned make 
it possible to separate the forms in question, and to erect them into the family 
Limacinkke, which, however, Souleyet unites with the typical Thecosomata. 2 
As to all the rest, they form a most natural family — Cavoliniidse, from which there is 
no reason to separate the genus Cuvierinci, under the name of Tripteridse, as Gray 3 and 
the brothers Adams 4 have done. The last mentioned genus in fact differs but very 
slightly from certain types of Cavoliniidse, such as the species of Clio of the section 
Creseis, from which indeed they are distinguishable only by the presence of a partition 
towards the middle of the shell, and by the constriction behind the aperture. 
Among the Thecosomatous Pteropods, we thus recognise only three families : — 
1. Limacinidse, including the genera Limacina and Peraclis. 
2. Cavoliniidse, ,, „ Clio, Cuvierma, and Cavolinia. 
3. Cymbuliidse, ,, „ Cymbulia, Cymbuliopsis, and Gleba. 
Family I. Limacinida 
1847. Limacinidx , Gray, A List of the Genera of Eecent Mollusca, their synonyms and types, 
Proc. Zool. Soc. Lond., p. 203. 
1859. Spirialidse, Chenu, Manuel de Conchyliologie, t. i. p. 113. 
Characters . — Shell external, twisted into a left-handed spiral, with a spiral oper- 
culum. Animal with a dorsal pallial cavity, and a ventral columellar muscle ; anus 
situated on the right side. 
Description . — The shell, which is always delicate as in other pelagic animals, is of 
small size, and is translucent with slight colouring. The spire and the operculum differ 
considerably in form in the various species. 
1 A Manual of the Mollusca, p. 204, 1856. 2 Histoire naturelle des Mollusques Pteropodes, p. 32. 
3 Catalogue of the Mollusca in the Collection of the British Museum, pt. ii., Pteropoda, p. 23. 
4 The Genera of Recent Mollusca, vol. i. p. 54. 
(zool. chall. EXP. PART LXV. 1887.) 
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