ATLANT. DEEP-SEA EXPED. 1910. VOL. III]. 



PTEROPODA. 



15 



when only full-grown specimens are used for comparison 

 (textfig. 11 B.), than when younger and older ones are 

 taken together (A. C); the overlapping of the two curves 

 is relatively small in the first case, while it gradually 

 becomes more pronounced with an increasing number of 

 younger shells. 



These statistical results prove clearly enough that the 

 two swarms of Limacina met with by the "Michal Sars" 

 Expedition at Stations 1 and 96 are really different from 

 each other with regard to the characteristic shape of 

 their shells. 



We then turn to the question whether any specific 

 structure characterises either of the two species to 

 distinguish them from each other. 



G. O. Sars (1878) in his detailed description of 

 Limacina balea and L. retroversa pointed to the stria- 

 tion of their shells as a specific character of high and 

 absolute value, while Boas (1886) does not acknowledge 

 this distinction. Although I agree with Sars in con- 

 sidering the two forms as distinct species, my results 

 with regard to the striation of the shells are more like 

 those of Boas. 



According to Sars the shells of L. balea have: 

 "Superficies subtillissime spiraliter striolata, striis regu- 

 laribus et aequidistantibus, ubique bene conspicuis," 

 while in the description of L. retroversa we find: "Super- 

 ficies laavissima, nitidula striis spiralibus parum con- 

 spicuis in anfractu ultimo omnino evanidis." This 

 difference he found to be constant in all stages of 

 development. 



After a minute investigation of the shells in several 

 of my samples I have come to the conclusion that 

 the existence of very fine spirally arranged striae is con- 

 stant in the shells of both species (see textfig. 12 A— B, 2, 

 where pieces of the shells are shown highly magnified), 

 while it will probably depend upon the fixation of the 

 material whether these striae will be conspicuous or not 

 when the shell is examined under a low power. The 

 material of L. balea at my disposal differs with regard to 

 the shell-structure in the various samples; in some samples 

 the spiral striae are, as described by Sars, very conspicuous, 

 while in others they are so fine that the longitudinal 1 ) 

 striping (or folding) of the whorls is the only shell-structure, 

 which attracts attention (textfig. 12 B. 1). In L. retroversa, 

 which form I know from one station only (St. 96), I have 

 found the spiral striae very conspicuous, even when seen 

 under a low power (textfig. 12 A. 1). 



The above statements will suffice to prove that the 

 striation of the shell is too variable a structure to give 

 any satisfactory character of distinction between the two 



') Parallel to the columella. 



species. I have therefore turned my attention to the radula 

 as the most trustworthy systematic structure of the molluscs. 

 The process of isolating and preparing the radula in 

 species as small as L. balea and L. retroversa is so 

 troublesome and difficult that distinctive characters drawn 

 from this organ will scarcely prove to be of great use 

 in practical systematic work. A demonstration of such 

 characters would, however, be of great importance as 

 giving the scientific justification for relying upon the less 

 exact but more easily recognisable characters of the 

 shell-form. 



After being boiled with caustic potash the radula is 

 in both forms seen to be of the common thecosomatous 



A 



B 



i 



Fig 12.. Shell-fragment of Limacina retroversa (A) and of L. balea (B), 

 slightly magnified (J), highly magnified .(2) 



type (formula 1 — 1 — 1), with apparently hook-shaped 

 teeth, the median ones symmetrically denticled on both 

 sides, while the lateral ones are denticled only on one 

 border. The radula of full-grown specimens of L. balea 

 is considerably larger than that of L. retroversa (compare 

 textfig. 13 with 14 B). 



In order to study the real shape of these minute teeth 

 it is necessary to isolate them from each other by a slight 

 pressure and then to examine each tooth by turning it 

 under the coverglass. The lateral teeth as well as the 

 median ones prove to be irregularly spoon-shaped, looking 

 very different when seen from different sides. 



