MOLLUSCA. 299 



HYALuEA TRIDENTATA. 



This mollusc, both in its own structure and that of 

 its shell, furnishes us with a link between univalve and 

 bivalve shell-fish. The shell is small and inclined to a 

 globular shape; and, although it has a broad and 

 permanent aperture in front, its sides betray a disposi- 

 tion to form two valves. It may be compared to a 

 bivalve shell without a hinge ; its posterior part being 

 consolidated, and armed with three short spines, while 

 its sides present a narrow fissure, through which a 

 membrane protrudes. The upper surface or valve is 

 longer than the lower, and projects forward, as a 

 hollowed ledge, or beak, tipped with gelatinous matter. 

 The entire structure of the shell is vitreous and semi- 

 transparent, and has a yellow-brown tint. 



The contained animal (a Pteropod) has no tentacula, 

 and its head is a mere rudiment ; its body is black, and 

 of soft consistence. A broad white membrane (branchia, 

 or " wing ") is attached to each side of the neck, and 

 protrudes through the aperture of the shell. We have 

 frequently seen the Hyalsea moving rapidly over the 

 surface of a calm sea by the aid of these wings, which 

 they employ as oars. Although they float passively in 

 their natural condition, when captured and put into a 

 basin of sea-water they invariably sunk to the bottom ; 

 and this they appeared to do by filling their shells with 

 water, which they could doubtless expel when they 

 wished to rise to the surface. 



We found this species very abundant on the surface 

 of the Pacific, from the Equator to 40 N. and 39 S. ; 

 as well as in the Indian Archipelago. 



