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is elongately triangular, its height being almost twice the length of the basis. Its 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 
margin, 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 l.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 carina and rostrum 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 beyond 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 flat and broad part at the hinder portion of the shell. The latter part is irregularly quadrangular, 
7 
