8i 



near the upper extremity of the valve. Rostrum narrow, partly covered by the lateral parts of 

 the rostral latera. Infra-median latus elongate, rhombiform, with the umbo near the base. Carinal 

 latus quadrilateral with the umbo quite at the base projecting beyond the line of the carina. 



A perfectly calcified specimen (PI. VII, fig. 9) of this form (form A) was collected at 

 Station 256, another (form C) very imperfectly calcified (fig. 11) at Station 45. I would not have 

 considered these two forms as belonging to the same species, but for the occurrence of a third 

 form (form B: fig. 10) — collected also at Station 45 — to a certain extent intermediate between 

 the two. A fourth specimen from the same Station 45 is exactly like that of fig. 11. It exhibits, 

 however, what the specimen I have figured does not, the margins of the valves, and shows 

 that these have exactly the shape of the corresponding valves in the other specimens I have 

 at my disposal. 



The capitulum is broad, oval, with the carinal side more strongly bowed than the 

 occludent .side. The apex is not very strongly recurved! The valves of the lower whorl increase 

 in height from the rostral to the carinal side of the capitulum. 



The scutum is perfectly calcified in all the specimens. It varies slightly in shape, as 

 is shown by the figures 9 — 11 of PI. VII, being at the tergal extremity narrower in the one 

 specimen than in the other. The occludent margin is more or less .strongly convex, the lateral 

 margin hollowed out near the upper extremity. The basal and lateral margins describe together 

 one continuous bow. The tergal and occludent margins meet at the apex and form a small 

 pointed projection which overlaps the tergum. 



The tergum is triangular. It has a convex occludent margin, whereas its carinal margin 

 is slightly hollowed out in the upper and slightly convex in the lower half of its length. The 

 scutal margin is nearly straight in the specimen with perfectly calcified valves; it is distinctly 

 concave in the intermediate form (fig. 10) and hollowed out over three quarters of its length 

 in the specimens with imperfectly calcified valves. 



The carina is rather strongly bowed; its roof is deeply notched longitudinally. Its sides 

 are broadest in the specimen with perfectly calcified valves and much narrower in the other 

 specimens. A broad stripe of chitinous membrane fills the interspace between the carina and 

 the other valves in these specimens. Another difference is that the umbo in the specimen from 

 Station 256 is quite at the apex, and at a little distance from the apex in the specimens with 

 less perfectly calcified valves. 



The upper latus is pentagonal with the umbo at the apex. Beyond the umbo the 

 apex is slightly more produced in the specimens with imperfectly calcified valves than in the 

 others. In the latter the two basal margins are straight, in the others these margins are 

 distinctly excavated. 



The rostrum (fig. 9a) is very narrow and on both sides covered by the rostral latera. 



The rostral latus is quadrilateral with the rostral margin much shorter than the lateral. 



The infra- median latus is rather large, broad and flat at the upper extremity, 

 much narrower at the base. The umbo is seated near the base at the tip of a triangular 

 projection, which is slightly raised over the narrow base of the valve. 



The carinal latus has a quadrangular shape when perfecdy calcified, with the carinal 



81 



SIBOGA-EXPEDITIE XXXI n. " 



