20 



KR. BONNEVIE. 



[REP. OF THE "MICHAEL SARS" NORTH 



With regard to the Limacinidae, we find in the 

 "Michael Sars" material representatives of all the species 

 previously known from the Northern Atlantic, viz. seven 

 species of the genus Limacina and three of Peraclis, 

 besides a new species of the genus Procymbulla. Some 

 of these species were found in quantities or under conditions 

 which allowed a deeper study of their systematical or 

 biological relations than had hitherto been possible. This 

 is true especially of Peraclis dlversa, Limacina helicoides, 

 L. balea and L. retroversa. 



With regard to the geographical distribution of the 

 Limacinidae we have found, besides the arctic L. hellclna, 

 one northern form, L. balea, belonging to the transition 

 zone between arctic and temperate waters and occasionally 

 following the cold water-currents down to more southern 

 latitudes (St. 66); all the other surface-species seem to 

 belong to warm or temperate waters. 



While most of the species of Limacinidae were taken 

 in depths less than 250 m., two species, viz. Peraclis 

 dlversa and Limacina helicoides, seem to belong to the 

 deeper layers of the ocean, the former however not 

 descending into the cold bottom-water. 



As a peculiar fact, which may in part depend upon 

 accidental circumstances, it may be mentioned that during 

 the "Michael Sars" expedition no species of Limacina 

 was found in the eastern part of the Northern Atlantic, 

 the European and African coast-line excluded. 



Cavoliniidae. 



The species belonging to this family are characterized 

 unlike those of the Limacinidae, by their (externally) 

 symmetrical body and the ventral position of their 

 pallial cavity. 



Modern authors (Pelseneer, Meisenheimer, Tesch) 

 seem to agree in the interpretation of Boas (1886) with 

 regard to the development of the Cavoliniidae from the 

 Limacinidae through a rotation of 180° of their body 

 relative to the head. There also seems to be a general 

 harmony with regard to the generic relations within the 

 Cavoliniidae, Creseis being considered as the most primitive 

 (Llmaclna-like) type of this family, while Clio and Ca- 

 vollnia represent the most extreme differentiation. The 

 agreement between different authors upon these points is 

 in fact so general, that I should consider it superfluous 

 to reopen the discussion of the question, if the results of 

 my investigation of the "Michael Sars" material had not 

 obliged me to look upon the relationship between the- 

 cosomatous pteropods in a way somewhat different from 

 that of earlier authors. 



As mentioned above, I have found in Limacina helico- 

 ides a species in which the dorsal pallial cavity occupies also 



the right side of the body, and among the Cavoliniidae 

 there is another species, Clio falcata, with its pallial cavity 

 lying ventrally but also on the right side of the body, 

 so that part of it is seen from the dorsal side of the animal. 



The existence of these two species belonging one to 

 each of the two great families of thecosomatous pteropods, 

 but representing at the same time progressive transition 

 stages between them, is of great general interest. On 

 the one hand it proves the correctness of the hypothesis 

 of Boas, in so far as the difference between the Cavo- 

 liniidae and Limacinidae is shown at least in part to be 

 caused by a rotation of the pallial cavity, but on the 

 other hand the additional information about these two 

 transition forms still living in the ocean forces one to 

 revise the questions as to what characters ought to be 

 considered archaic and what characters are modern, and 

 therefore also as to the relationship between different 

 genera of each family 



These questions will be fully discussed later, and at 

 present I shall only use the results of my investigation 

 in arranging the genera and species according to the 

 supposed relationship between them. Primitive characters 

 are found above all in different species of Clio, but also 

 in Diacrla, and partly in Cuvierlna; I therefore consider 

 these groups to be the starting points of diverging lines 

 of differentiation within the family Cavoliniidae, the limits 

 of which agree with those of the genera Clio, Cuvierlna 

 and Cavollnla of Pelseneer. I shall therefore also maintain 

 his division of the Cavoliniidae into genera and subgenera 

 in spite of Meisenheimer's proposal to consider all the 

 different groups as equivalent genera. 



Clio. 

 Subgenus Euclio (Clio) Linne. 



Of this group I have investigated three species: Clio 

 falcata, C. cuspidata and C. pyramidata, which, besides 

 the dorso-ventral compression and the lateral carinae of 

 their shells, have also a number of other characters in 

 common. Among these I shall here only mention the 

 existence in all of them of a pointed triangular lobe 

 above the mouth and a certain asymmetry of their 

 pallial cavity. 



Clio (Euclio) falcata Pfeffer. 



PI. II, fig. 12-13, 16—20. 



Cleodora falcata, Pfeffer, 1880 (p. 96, fig. 19). 

 Boas, 1886 (p. 80). 

 Munthe, 1887 (p. 20). 

 Clio polita Pelseneer, 1888 (p. 60, pi. II, fig. 4-6). 

 Meisenheimer, 1905 (p. 20). 

 — falcata Meisenheimer, 1906 (p. 422, fig. 4). 

 Lenz, 1906 (p. 5, fig. 1—3). 



