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roof above it. The growth-ridges on the different valves are not very prominent, on the rostrum 
only they are rather distinct and broad. 
The moveable scutum has the apex pointed, the tergal margin hollowed out near 
the apex, and slightly protuberant at the basi-tergal angle; the basal margin is straight and the 
occludent margin strongly curved. The main articular ridge is distinctly curved, a second ridge 
runs closely to it, separated from the tergal margin by a flat, narrow part, without a trace of 
an articular ridge along the tergal margin. 
The moveable tergum has the apex slightly produced, but obtuse and not pointed ; 
its shape is nearly regularly rhomboidal with the longer occludent margin curved and the shorter 
slightly hollowed out. The basal margin is straight, the scutal straight also, with a feeble 
protuberance near the apex of the scutum and another near the basi-scutal angle. The main 
articular ridge extends from the apex to the basi-scutal angle and is rather strongly curved; 
another ridge extends along the shorter occludent margin. 
The carina is narrower but perhaps a little higher than the rostrum, its apex is pointed 
but can hardly be said to project freely. The lateral margin is feebly curved and is visible at 
the side of the fixed scutum and tergum only; the rostral margin shows a distinct excavation 
near the upper extremity, for the reception of the rounded tooth on the carinal margin of the 
rostrum. A triangular portion of the surface of the valve near the tergal margin develops into 
a slight swelling which forms the continuation of the more pronounced swelling on the surface 
of the rostrum. 
The rostrum has the lateral margin shorter than the carinal; while the former is 
perfectly straight, the latter has a distinct excrescence near the upper extremity, and forms the 
rather broad tooth with rounded circumference which fits in the excavation of the rostral margin 
of the carina. A flat semilunar part of the surface is to be seen along the scutal margin of 
the valve and is separated from the rest of its surface by a distinctly swollen part which grows 
broader and more prominent towards the carinal margin, where it terminates into the tooth 
already mentioned. 
The fixed tergum is larger — broader as well as higher — than the fixed scutum. 
Its occludent margin consists of two parts which closely correspond to the two occludent 
margins of the moveable tergum; its apex is indistinctly beaked. The valve itself consists of a 
middle portion and two lateral portions. The middle portion is triangular and curved and 
terminates in the apex; the lateral portions are triangular also: their upper margins are each 
formed by one of the parts of the occludent margin of the valve. At the side of the carina 
the lateral portion is overlapped by a narrow strip of that valve, at the side of the fixed 
scutum by a kind of radius belonging to the latter valve. 
The shape of the fixed scutum is irregularly triangular, broad at the base, pointed 
at the apex. It overlaps the fixed tergum with a triangular portion representing a radius. This 
portion of the valve is broadest near the upper extremity and very narrow at its base; its 
upper margin is hollowed out in a very characteristic manner. The remaining part of the valve 
consists of the curved and triangular middle part and a lateral part of triangular shape also, 
the free margin of which is attached to the lateral margin of the rostrum. 
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