EXPEDITION ANTARCTIQUE BELGE 



The movable scutum is rather large, though not inconsiderably smaller than the movable 

 tergum. It has the ordinary triangular shape. Its occludent margin is curved and forms with the 

 tergal margin a pointed, slightly beaked apex. The basai margin is straight ; its length equals 

 two thirds of the length of the tergal margin. The latter describes an irregular Une : in its 

 under part it has two excavations, in which fit the extremities of the articular ridges of the 

 tergum, and between thèse two excavations the one ridge of the scutum forms itself a tooth- 

 like projection ; the upper part of the tergal margin shows a little protubérance which fits into 

 a shallow excavation of the scutal margin of the tergum. The scutum has two well developed 

 articular ridges which run from the basi-tergal corner nearly to the apex of the valve. The 

 growth ridges run parallel to the basai margin in the main part of the valve and nearly parallel 

 to the tergal margin in the smaller part which is situated between one of the articular ridges 

 and the tergal margin. 



The movable tergum is broad and its shape is irregularly quadrangular. Two of its sides 

 form together the occludent margin and meet in a rounded angle ; the third side is the nearly 

 straight basai margin, the length of which equals that of the scutal margin. The latter fits with 

 its teeth and excavations into the corresponding excavations and teeth of the tergal margin of 

 the scutum. The surface of the valve is furnished with two distinct ridges : the axial ridge is 

 the most prominent, it widens downwards and projects distinctly at the scutal-basal point ol 

 the valve. The other articular ridge runs close to the axial ridge ; it is less prominent but it 

 widens also downwards. The part of the valve situated between the latter ridge and the occlu- 

 dent margin is slightly hollowed out but there is no trace of a third ridge in this part of the 

 valve. 



FlG. 2 



Same animal, seen from the side of the fixed scutum and tergum. Magnified 10 diameters. 

 s' = fixed scutum ; f = fixed tergum ; r = rostrum ; c = carina. 



The fixed scutum is shorter, but considerably broader towards the base than the tergum. 

 It consists of two triangular portions meeting along a curved Une and describing together an 

 obtuse angle. Both portions show very distinct lines of growth and are beaked at the apex. 

 The occludent margin is curved and furnished with a slightly upturned marginal rim ; the 

 terginal margin is hollowed out and furnished with a kind of radius projecting over the edge 



