264 Contribution to our knowledge of Pelagic Hollusca. [No. 4, 



temperature of the water generally is lower than that of the air. 

 From the 8th degree of North Latitude in West Longitude to the 

 Equator in the Indian Ocean the water is on the contrary warmer 

 than the air. 



The direction of the several currents in the table (p. 269) is taken 

 from Beecher's sailing directions for the Atlantic and Indian Oceans. 



Note on Hyal^a tridentata, Lamck. 



With regard to this shell, M. Rang in his work on Pteropods* 

 says that it presents two varieties, distinguished by their smaller 

 size ; the animal and shell of one being darker in colour, the other 

 having a very clear, and almost transparent yellow colour. To the 

 first of these varieties, he refers H. affinis of D'Orbigny, but it is not 

 clear to what he refers the second ; however, as he elsewhere objects 

 to Peron's H. teniobrancliia being regarded as a separate species, the 

 inference is, that he considers it identical with his second variety ; al- 

 though, in truth, it is not distinguished from H. tridentata by its 

 smaller size. 



In explanation of his text, he gives the following illustrations: 



first, H. Forslcaldii, D'Oib., pi. ii, figs. 1 to 5. 



second, H. tridentata, Lamck., pi. xii, figs,l to 4, 

 (figured by Souleyet, both being representations of the same shell ; the 

 animals of the first, however, being incorrectly drawn,) and 



third, H. teniobrancliia, Peron, 

 but no figure is given of the variety he refers to H. affinis, D'Oib. 



Having been so fortunate as to capture the three shells standing at 

 the head of the list in the table (p. 269), and been able to examine 

 them both in their living and preserved state, I am persuaded that 

 H. tridentata, Lamck, H. teniobrancliia, Peron, and H. affinis, D'Orb., 

 represent three separate and distinct species, and I think this will 

 be seen, on referring to plate xxi, where front (la, 2a, 3a), back 

 (lb, 2b, 3b), and side (lc, 2c, 3c) views of them are shown. f 



In their front aspect, that is to say, with the ventral plate uppermost 



* Hist. Nat. des Pteropodes, 1852. MM. Rang and Souleyet. 



f Figs. 2 and 3, plate xxi, are copied from drawings from nature by Dr. 

 Ramsay Stewart, R. A., a fellow passenger on board the Superb to whom 

 my best thanks are due. 



