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is elongately triangular, its height being almost twice the length of the basis. lts occludent 

 margin is curved, its basal margin straight, its tergal margin straight also, yet furnished with 

 numerous indentations. The outer surface shows numerous articular ridges, the most pronounced 

 of which, the axial articular ridge, describes a distinct arch and runs from the basi-tergal angle 

 to the apex. Four other ridges occupy the interspace between the axial ridge and the tergal 

 maro-in, and run parallel to each other, and each following one is shorter and less strongly curved 

 than the preceding one. Each ridge, moreover, grows slightly broader towards the under extremity 

 and is distinctly divided by transverse lines corresponding with the growth ridges of the valve 

 itself. The fourth of these ridges is the shortest ; it is nearly straight and runs along the tergal 

 margin of the valve. 



The moveable tergum is large and quadrangular. Of the occludent margins the 

 longer is feebly curved and the shorter slightly hollowed out ; they meet at the apex which 

 is distinctly beaked. The scutal margin is nearly straight, yet furnished with indentations in 

 which fit the teeth of the tergal margin of the scutum ; the basal margin is nearly straight, 

 or, perhaps, feebly curved, but 1. c. this could be decided only by isolating the different valves, 

 which I have not thought desirable. The outer surface of the tergum shows a strongly developed 

 and curved axial articular ridge, which widens considerably towards the scuto-basal angle ; it 

 has moreover a distinct marginal ridge along the shorter occludent margin and five intermediate 

 ridges nearly filling up the whole interspace between the two first described. These five ridges 

 run nearly parallel to the axial ridge and to each other ; from the apex down to the scutal 

 margin of the valve each ridge grows wider and slightly diverges ; each of their extremities forms 

 one of the teeth for the articulation of this valve with the moveable scutum. All these ridges 

 are transversely divided by furrows which correspond with the growth ridges on the surface of 

 the remaining part of the valve. 



The c a r i n a and rostru m have both an irregular quadrangular shape, the carina 

 being a little higher, the rostrum a little broader. The apices of both valves project freely, and 

 slightly bevond the edge formed by the other valves. The carina is furnished with a distinct 

 furrow, running at some distance from the tergal margin towards the apex of the valve. At 

 the inferior extremity of this furrow an indentation is seen on the rostral margin, in which fits 

 the tooth-like point of the rostrum at the scuto-carinal angle of that valve. For the rest the 

 suture between the rostrum and carina is straight and has no teeth or indentations. The surface 

 of the rostrum is divided into two triangular parts by a rim running from the basi-carinal 

 margin to the apex : from this rim the lines of growth run parallel to the carinal margin in 

 the upper part and parallel to the basal margin in the lower part. The surface of the latter, 

 as also that of the part of the carina which lies beneath the furrow, and more distinctly so 

 towards the basal margin of the valve, is not smooth, but folded, the folds being perpendicular 

 to the growth ridges. 



The fixed scutum and tergum have also a very characteristic structure. The fixed 

 tergum is composed of two parts: a triangular portion which is very narrow at the apex and 

 slightly broader in its inferior half where it articulates with the lateral margin of the carina, and 

 a fiat and broad part at the hinder portion of the shell. The latter part is irregularly quadrangular, 



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