THE BAENACLES IN THE U. S. NATIONAL MUSEUM. 



63 



The scutum is about twice as long as wide, rhomboidal, with acute 

 apical umbo. The occludent and lateral margins are subparallel and 

 somewhat convex. The tergal margin is concave, the basal straight. 



The tergum is triangular, conspicuously larger than the scutum. 

 The occludent margin is about three-fourths as long as that of the 

 scutum and is strongly arched, the acute summit being thus somewhat 

 recurved. The scutal margin is a little longer than the occludent; the 

 carinal margin is a little hollowed near the apex, elsewhere convex. 



The carina is long and equably curved, its apical umbo at about the 

 upper fourth of the carinal margin of the tergum. The roof is flat 



Fig. 25.— Scalpellum semisculptum x 3, with details of rostrum b, and carina, c. 



between low but strong bordering ribs. The sides are narrow below, 

 wider above; basal margin convex. 



The upper latus is trapezoidal, the scutal margin concave, carinal 

 margin very short, about one-third the length of the scutal. The 

 tergal and basal margins are about equal and straight. The umbo is 

 terminal; a narrow rib runs from it to the baso-carinal angle. The 

 plate is finely, rather sharply striate radially. 



There is no rostrum, or merel}^ a sunken linear rudiment. The 

 rostral latus is as high as wide, the basal margin very much shorter 

 than the others; scutal margin straight, horizontal. The umbo pro- 

 jects a little at the upper occludent angle. From it a narrow diagonal 

 rib runs to the lowei' lateral angle, the surface below this rib being 

 radially striate. The two rostral latera rise in a low welt or irregu- 

 lar ridge at their occludent suture. 



The inframediau latus is very narrow, almost linear, and about as 

 long as the adjacent border of the rostral latus. It curves above 



