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



PTEROPODA. 



13 



For the same reason Pelseneer (1888) also proposes 

 to unite the two forms into one species. 



About ten years later, however, Locard (1897) once 

 more describes the two forms as distinct species. He 

 says (pag. 25): "Nous distinguerons done le Limacina 

 balea du L. retroversa: a son galbe beaucoup plus etroite- 



ZTH 



dd. 



Fig. 9. For explanation see text. 



ment allonge, de telle sorte que pour un meme diametre 

 la coquille du Limacina balea est toujours plus haute; 

 a sa spire composee de tours plus nombreux, croissant 

 plus lentement en diametre et plus rapidement en hauteur; 

 a son dernier tour notablement plus haut et plus £troite- 

 ment arrondi; a son sommet plus acumine; a son ouver- 

 ture toujours plus haute que large, et non pas plus 

 large que haute, etc". 



But Locard is not followed by later authors 

 (Posselt'1898, Meisenheimer 1905, 1906, Lenz 1906), 

 who again describe the two forms as varieties of 

 one and the same species. 



A definite solution of the question about the 

 relations between L. balea and L. retroversa may be 

 reached in two different ways: — 



1) through a statistical investigation with regard 



to the value of the transitions in the shape of the . 



shells combining the two originally described forms 

 with each other. Do these intermediate forms prove 



that L. balea and L. retroversa are representatives of one 

 and the same species? Or are they to be considered as 

 the fluctuating variations of two species partly overlapping 

 each other? 



2) through a demonstration in the shell or in other 

 organs of some minute structures by which the two forms 

 can be specifically distinguished and characterised. 



I have tried both ways, and both have given the 

 same positive result, namely that the two forms so inti- 

 mately intermingled in literature are yet distinctly separated 

 from each other in nature. 



During my first preliminary investigation of the 

 material I — following Boas and Pelseneer — considered all 

 samples as belonging to one and the same species, and 

 I noted that immense swarms occurred at no less than 

 8 stations, two of which, St. 1 and 96, were taken near 

 each other off the Irish coast, but with an interval in 

 time of about 3Va months ( 9 /*— 27 / 7 )- Already at the first 

 glance, however, these two samples differed very con- 

 spicuously from each other, the sample from St. 96 con- 

 sisting of individuals considerably smaller and darker than 

 those of St. 1 and all other stations. A further comparison 

 of these samples made me doubt the correctness of the 

 opinion maintained by Boas and Pelseneer, and therefore 

 they were made the basis of the following statistical in- 

 vestigation, the material of St. 1 as representing the form 

 L. balea Mailer, that of St. 96 representing L. retroversa 

 Flemming. 



Textfig. 9 A shows three stages in the development 

 of the shell of L. balea, while in B. the corresponding 

 stages of L. retroversa are drawn on the same scale. The 

 difference between the two forms is to be found, as has 

 been pointed out by earlier authors, in the number of 

 whorls of the full grown specimens as well as in the size 

 of the last whorl compared with the length of the spire. 

 Slight and relative as the latter character may be, it 

 can be traced through all stages of the two forms, the 

 difference being, however, less conspicuous between young 



Fig. 10. For explanation see text. 



shells than between older ones. As will be seen from 

 the figures the absolute size of the two forms is also 

 different, L. balea being larger than L. retroversa. 



Within each species there is a considerable variation 

 with regard to the slenderness of the shells, the larger 

 individuals, especially those of L. balea, being relatively 

 more slender than the smaller ones. As will be seen from 



