THECOSOMATA de Blainville 



1821. Thecosornata de Blainville, Dict. des Sc. Nat., t. XXXII, p. 271. 



1885. Pterocepliala [part.) Wagner, Die Wirbellosen des Weissen Meeres, Bd. I, p. 119. 



1886. Eiipteropoda Boas, Spolia Atlantica, p. 14. 



The great number of genera, in which the species of the Thecosornata were divided, has 

 been reduced to eight by Pelseneer ^). These genera belong to three families, which, especially 

 with respect to the shape of the shell, differ sufficiently from each other. to be distinguished 

 at first sight. They are Limacinidae, CavoHniidae and CymbuHidae. 



The external parts of the body of the Thecosornata : the shape of the foot, of the fins, 

 of the tentacles, etc, have been described so often and in details, that it does not seem 

 necessary to repeat what has been said on this subject by Souleyet"), Boas^) and Pelseneer*) — 

 neither have the general anatomical characters need to be spoken of. 



What has been published about the anatomy by van Beneden ^), Gegenbaur ''), 

 Souleyet '), Boas ■) and others, was still importantly augmented by Pelseneer by an exact 

 anatomical study of each genus separately ''). Pelseneer gives thirty-four generic names. To 

 my opinion two other names ought to have been mentioned among these: Anomia Forsk&l 

 and Gioeiiia Gioëni (both -= Cavolinia Abildgaard). This last genus I can only mention on 

 the authority of de Blainville (article Hyale in Dict. d. Sc. Nat. XXII) and of Souleyet 

 (Hist. Nat. Ptérop. p. 33, note i), as I have not had the opportunity to see Gioëni's work 

 myself. The species of the genus Agadina have been recognised by Pelseneer himself as 

 Gastropoda-larvae (Streptoneural). 



Through these investigations it has been sufficiently proved: 



1" that the Thecosornata descend from the BuUoidea, 



2" that the Limacinidae are the original forms, 



3" that further specialisation with the intention of accommodating to the pelagic way ot 



living, has led to the apparent (external) symmetry, which appears in most forms of the 



Cavoliniidae and 

 4" that these two families do not descend from each other, but that the external and internal 



characters of the genus Peraclis among the Limacinidae are connected to each other on the 



one side by Limacina with the Cavoliniidae, on the other by larval stages of Cyuibtilia 



with the Cymbuliidae. 



The question in how far there is any connection between the dorsal mantle hole of the 

 Limacinidae and the ventral one of the Cavoliniidae — which is accompanied by a great many 

 other differences between the two families — has been spoken of by Souleyet and Grobben. 



1) Challenger Report. LXV, p. 5—8. 



2) Voyage de la Bonite. VoL II, 1852. 



3) Spolia Atlantica. 1885. 



4) Challenger Report. LXV, LXVI, 1888. 



5) Exercices Zoötomiques. Mém. Acad. Sc. Brux. 1841, Vol. XII, 1839. Mém. sur la Limacina arctica. Op. cit. Vol. XIV. 



6) Untersuchungen über Pteropoden und Heteropoden. Leipzig 1885. 



7) Challenger Report. LXVI, p. 5 — 37. 



