ECOLOGY— DEPTH DISTRIBUTION i75 



much less. It is perhaps worthy of note that L. helicoides, by far the largest species of Limacina 

 (12-5 mm.) is also the only one found regularly at great depths (see Tesch, 1948). 



It follows that at the night station WS 997, worked between 1 and 4 a.m., we should not expect to 

 find a great concentration of the population at the surface. Those species accustomed to a high con- 

 centration of light should lose the impulse to come to the surface soon after dark, and would have 

 begun to sink downwards by the early hours of the morning. Such a species will probably rise again 

 early with the dawn, but hauls between 1 and 4 a.m. may be expected to coincide with the period of 

 night when the stimulus to rise is weakest. From the greatest concentration by night at the 100-250 m. 

 level, L. bulimoides would seem to show little upward movement— perhaps even a descent, from its 

 day-time optimum. The greatest depth at night, however, is still less than that of any cavolinnd 

 optimum (except Creseis virgula) by day, and the population of Limacina as a whole is probably not 

 significantly responsive to light at all. There is an apparent tendency for a minority to rise to the 

 surface at night. It is probable that only the larger individuals and strongest swimmers will be able 

 to make this migration, and that these individuals will come not from the 250 m. level but from water 

 layers of less than 100 m. depth. Further, the individuals at the surface at night may be expected to 

 be mainly large ones. 



How does the size composition of the population at various depths at WS 997 agree with this con- 

 clusion? By reference to the histograms (Fig. 3) for the levels 0-50 m. and 100-250 m., a great falling 

 off is found in the percentage of small individuals near the surface— twenty-one as compared with 

 forty-one for the added percentages of the two lowest size groups. Conversely there is an increase in 

 the percentage of larger individuals at the top, thirty-five as compared with twenty-five for the added 

 percentages of the two largest size groups. In the more homogeneous day-time population at WS 996, 

 the comparative percentages are twenty-four for the two large groups and twenty-six for the two 

 smallest groups. It may thus be possible to explain the constriction in the distribution diagram for 

 WS 997; there is a tendency to depth division of the population at night, larger individuals tending to 

 rise nearer to the surface. The bulk of the population including smaller individuals and those present 

 in the deeper layers of the day-time distribution, are not able to reach the surface at night, and show 

 little tendency to migrate at all. Perhaps with no light stimulus to hold them at day-time level, they 

 tend to sink a short distance, which they do faster and more easily than they are able to compensate 

 by swimming up. It would be interesting to look for similarly constricted distribution figures for the 

 night behaviour of other planktonic organisms relatively feebly responsive to light. 



Superfamily Euthecosomata 

 Family Cavoliniidae 

 Superfamily Pseudothecosomata 

 Family Cymbuliidae 

 These two families, constituting the larger size thecosomatous pteropods, were rather poorly repre- 

 sented among the mollusca of the first survey, the first by small numbers of juvenile individuals of 

 three species, and the second by a single juvenile individual. 



Diacria trispinosa (Lesueur) 



1821 Hyalaea trispinosa Lesueur, in de Blainville, Diet, de Sci. Nat. xxu, 65-83. 



1905 Diacria trispinosa Meisenheimer, Pteropoda, Wiss. Ergebn. D. Tiefsee-Exped. 'Valdivia', ix, 27. 



1946 Diacria trispinosa Tesch, Dana Reports, xxvm, p. 24, pi. iii, fig. 14. 



This species occurred only at WS 986, represented by three individuals at 50-100 m., and two at 



100-250 m. The three first specimens are the young form, the Cleodora compressa of Souleyet, 



