THECOSOMATA. 



Since the important papers of Boas (1886) and of 

 Pelseneer (1888) who, though working independently of 

 each other, reached very much the same results with 

 regard to the system of thecosomatous pteropods, this 

 system has undergone no essential changes. Later in- 

 vestigators, like Tesch (1904) and above all Meisenheimer 

 (1905, 1906) have, however, made very valuable additions 

 to our knowledge of the natural relationship between 

 the various groups of the system. 



On a few minor points only the system of Meisen- 

 heimer differs from that of (Boas and) Pelseneer, especially 

 with regard to the grouping of the Cavoliniidae, Pelseneer 

 dividing this family into three genera, one of which, Clio, 

 is again divided into a number of subgenera, while 

 Meisenheimer finds it more correct to give all these sub- 

 genera generic rank. At the same time Meisenheimer 

 maintains a grouping within the families of thecosomatous 

 pteropods, connecting the two families Limacinidae and 

 Cavoliniidae under the name of Euthecosomata, while 

 the third family, Cymbuliidae is considered a representative 

 of another group, the Pseudothecosomata. 



Pelseneer in his short paper on "Biscayan Plankton" 

 (1906) has extended our knowledge of one genus of the 

 family Limacinidae, viz. Peraclis. 



As will be seen from my description of the material 

 of thecosomatous pteropods taken by the "Michael Sars" 

 Expedition 1910, I have in the systematic grouping of 

 the species followed the line adopted by earlier authors; 

 with regard to the natural relationship between different 

 groups (genera and families), however, my view differs 

 essentially from that maintained in earlier papers. This 

 difference is based upon comparative anatomical results 

 especially regarding a few deep-sea species, and will be 



thorougly discussed later on. At the same time the reason 

 will be given also for my following Pelseneer with regard 

 to the division of the Cavoliniidae into genera and sub- 

 genera, in spite of the proposal of Meisenheimer of giving 

 each of these groups generic rank. 



The geographical distribution of thecoso- 

 matous pteropoda has been so thoroughly treated by 

 Meisenheimer (1905, 1906) that very little remains to be 

 said. The rich material of the "Michael Sars" Expedition 

 will, however, be of value, as giving the means of testing 

 in a limited area the correctness of the general views set 

 forth by Meisenheimer. 



My treatment of the different species will be found 

 very unequal with regard to synonyms, as well as to 

 diagnoses and descriptions of anatomical characters. The 

 reason is that I have not found it necessary to repeat the 

 whole series of indisputable facts already fully discussed 

 in earlier papers, while I have paid much attention to 

 the clearing up of some questions with regard to which 

 earlier authors do not agree, or where my own results 

 have proved to be contradictory to theirs. 



Special attention will in the systematic part be paid 

 to the genus Peraclis, the species Limacina hellcoides, 

 balea and retroversa, and- Clio falcata, while for most 

 of the other species nothing will be done beyond stating 

 their distribution within the region investigated by the 

 "Michael Sars", and some biological facts regarding their 

 shape or occurrence (Clio pyramidata, Diacria trispinosa, 

 Cavolinia inflexa). 



The textfigures showing isolated radula-teeth of the 

 different species are all drawn on the same scale (Obj. 5, 

 Oc. 3) so that they may serve as a demonstration not only 

 of the shape but also of the relative size of the teeth. 



