247 



the tergum between the specimens from the different stations, there could be no doubt that 

 they belonged to the same species. 



The shell is white and covered by a greyish membrane with a velvety surface. This 

 membrane is quite persistent round the basal part of the shell, the surface round the orifice being 

 almost quite bare. The orifice is large, of a pentagonal shape ; it is toothcd in consequence 

 of the oblique summits of the alae. This is more distinctly the case in the specimen of cylin- 

 drical shape from Station 105. The oblique summits of the alae are delicately crenated, their 

 surface is horizontally striated. The carina and the carino-lateral compartment have the alae, in 

 the cylindrical specimen especially, very broad at the summit. The ala of the lateral compartment 

 is also of a triangular shape, but it is much narrower in the upper part. The rostrum has 

 neither radii nor alae; it is of a triangular shape, bluntly pointed at the tip, broad at the basis 

 and very broad in the specimens from Stations 59 and 251. The outer surface of all the 



Fig. 1. Outer view of the valves 



of Hixclasma veluiinum gen. et spec. nov. 



RO, Rostrum; CA, Carina; LA, Lateral; 



CL, Carino-lateral compartment. 



Fig. 2. Inner view of the valves 



of Hexelasma veluiinum gen. et spec. nov. 



RO, Rostrum: CA, Carina: LA, Lateral: 



CL, Carino-lateral compartment. 



valves is striated horizontally, the striae being more distinct on the rostrum and the carina 

 than on the lateral compartments. On the inner surface the compartments show very delicate 

 longitudinal lines, but these are not to be confounded with longitudinal ribs or septa, of which 

 there is no tracé. The sheath of the rostrum (PI. XXVI. fig. 6) is developed in the middle 

 of the valve onJy, the part of the valve on each side of it being slightly excavated for the 

 reception of the alae of the rostro-lateral compartments. The compartments seem to hang 

 together in a very loose way: on manipulating a specimen it easily falls to pieces. The basis 

 is membranous. 



With the aid of the specimen from Station 59, I was enabled to study the shape of 

 the different valves more in detail. The accompanying figures show these, 4 times enlarged. 



119 



