1869.] Contribution to our knowledge of Pelagic Ilollusca. 265 



figs, la, 2a, 3a, the contour of the first is semi-spherical, the second 

 subtriangular, the third somewhat of an elongated oval. 



Viewed inferiorly, figs, lb, 2b, 3b, the sculpture of the dorsal plate 

 in each, presents the following peculiarities. — 



In JEL. tridentata, there are five longitudinal ribs, and, as Mr. Benson 

 very accurately observes " the edges of the lateral rifts are expanded 

 and very thin, and on the flatter side* are wrinkled at right angles to 

 the line of the rift and slightly reflected at the edge."f 



In H. teniohranchia three longitudinal ribs converge separately, 

 and narrowing as they approach the terminal tooth, unite above it. 

 At the lateral points the edge of the ventral plate overlaps the dorsal ; 

 at the shoulders the dorsal plate overlaps the ventral. 



In S. affinis, three longitudinal ribs unite in the upper half of the 

 dorsal plate and thence descend to the terminal tooth in one more or 

 less well defined prominent rib. 



The side view shows the degree of convexity in the ventral plate of 

 each shell. 



In figures, 1,J 2,§ 3, we see differences also in the animals. In 

 fig. 1, the alar appendages are rectangular and trilobated at their 

 external edge ; In fig. 3 they are oval ; In fig, 2, they are of inter- 

 mediate shape. 



The lateral expansions of the mantle, in H. tridentata and H. affinis^ 

 are both broad and short. In the latter they are yellowish towards the 

 extremity and dark brown at the tips ; the buccal mass too is of a dark 

 brown colour. In H. teniohranchia on the other hand, the mantle 

 processes are filiform, in length about two inches, extremities yellow, 

 with dark brown tips. 



As Dr. J. E. Gray in his catalogue|| has given no description of 

 H. teniohranchia, and only a brief one of H. tridentata, I append 

 a description of each, recasting the one given by him of H. qffinis and 

 adding his synonyms of H. tridentata. 



* i. e. dorsal. 



f Mr. Benson's notes on Hyalcea, Vol. 7. Ann. Mag. Nat. Hist. 3rd Series, 

 No. 37. 



X After Souleyet. 



§ After Peron and Lesneur. 



|| Catalogue of Pteropoda in the British Museum, London, 1850. 



34 



