149 



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 scu t u m 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 tracé of 

 an articular ridge along the tergal margin. 



The moveable ter gum 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 

 maro-ins 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 marmn of which is attached to the lateral margin of the rostrum. 



