THE BARNACLES IN THE U. S. NATIONAL MUSEUM. 



65 



strai^'ht soutal margin. The carinal margin is ver}- weakly sigmoid, 

 concave abov^e, convex below. The umbo is a trifle recurved. 



The carina is simply arched, more strongly so above; umbo termi- 

 nal. The roof is flat with distinct bordering ribs. The sides are 

 moderately developed near the umbo, but very narrow elsewhere. 

 The basal margin is straight, as are the fine lines of growth across the 

 roof. 



The upper lateral plate is trapezoidal, the scutal margin much the 

 longest, concave. The other margins are straight, the carinal shortest, 

 less than half as long as the scutal. The apex is produced in a small 

 triangle above and beyond the umbo, which is acute and marginal, on 

 the scutal side. 



There is no rostrum. 



The rostral latus is quadrangular, about as high as wide. The ven- 

 tral and scutal borders are straight, the basal very short. The upper 



Fig. 26.— Scalpellum aurivillii. o, b, lateral views x 3; c, S. aurivillii, in'certvm lateral 



VIEW X 2. 



interior angle is truncated where it comes in contact with the upper 

 latus for a short distance. The carinal margin is in contact with the 

 carinal latus, but the suture is more or less covered by the extremely 

 narrow inframedian latus, which overlies the borders of the plates. 



The inframedian latus is very narrowly triangular, being wider at 

 the base. The umbo is apical. It overlies the suture instead of occu- 

 pying a space between the rosti'al and carinal latera. It is often 

 abnormal, as noted below. 



The carinal latus is twice as high as wide, quadrangular, the umbo 



at its lower third not projecting beyond the carina. The basal and 



rostral margins are subequal, straight, and at right angles. The 



carinal margin is nearly straight, projecting a little in the lower third. 



4715— Bull. 60—07 ^ 



