32 Mr. W. H. Benson on the Pteropodous Genus Hyalsea. 



species) then took its place as far as 4° and 5° N. lat. and 90° 

 E. long. 



Its capture was recorded on eleven different days ; and the 

 largest number of specimens taken on a single occasion occurred 

 on the night of December 1 (in 33° S. lat. and 81° E. long.), 

 when forty-five examples of the larger variety were secured, in 

 three towing nets, in the course of a few minutes after darkness 

 had set in. 



The large variety agrees with the figures given by Rang in 

 plate 3 as H. Forskalii (asserted by Souleyet to be H. tridentata, 

 Lamk.), and with the description given by Souleyet of the latter 

 species, as well as with Krauss's description and figure of his 

 H. truncata from False Bay, in the ' Siid-Afr. Moll./ — the speci- 

 mens taken up to the 1st of December being of a clear horn- 

 colour, and having the gibbous portion in front and the summit 

 of the labrum tinged with a darker hue in most specimens; 

 while another variety, of the same size and characters (of which 

 I took three specimens in 31° S. lat. and 83° E. long.)^ presents 

 the colouring assigned by Krauss to his H. truncata. The 

 greatest length is 18 mill., with a breadth of 12^ mill. 



The smaller variety, taken near the Equator in the Indian 

 Ocean, differs in its narrower and less elevated labrum not 

 emarginate in the centre, and in having only three broad ribs 

 on the flatter face of the shell, instead of five assigned by Sou- 

 leyet to H. tridentata, and by Krauss to H. truncata ; also in the 

 absence of radiate striae on the ventral side. It may possibly 

 be the species which Krauss viewed as H. Forskalii when he de- 

 scribed H. truncata as distinct. I shall not risk an increase of 

 the evident confusion already caused by misapprehensions con- 

 cerning the true characters of the original species. This can 

 only be cleared up by reference to authentic examples ; but the 

 following characters may help to solve the difficulty. I can find 

 no allusion to them in any work to which I have access. 



In the larger five-ribbed species the edges of the lateral rifts 

 are expanded and very thin, and on the flatter side (called the 

 dorsal by Souleyet and Kraviss, and the lower by Mdrch) are 

 wrinkled at right angles to the line of the rift, and slightly re- 

 flected at the edge ; while in the smaller three-ribbed shell the 

 edge of the rift, instead of being expanded, is thickened by a 

 rib on the ventral side, and the dorsal edge is inverted into the 

 rift. This seems to be a very decided distinction, and, in con- 

 junction with the fewer ribs and the structure of the labrum, &c., 

 may justify a specific separation. 



The dimensions of three specimens of the smaller shell are — 

 Length 14, breadth 9 mill, (two specimens). 

 „ 12, „ 8 „ (one specimen). 



