THE SESSILE BARNACLES. 



225 



porous; rostrum and lateral compartments usually parted into two 

 or three broad, low, rounded ribs near the base, by one or two shal- 

 low furrows. Eadii broad. Orifice pentagonal, with two long sides; 

 closed by the scuta, which lodge high and project above the lateral 

 compartments. 



Scutum (pi. 48, h'gs. 2c, d] slightly arched from base to apex, the 

 outer face with a longitudinal flattening or slight depression, marked 

 with low growth-lines but no longitudinal strise; occludent edge 

 crenated; a tergal segment rather deeply deflected. Inside there is a 

 moderately high articular ridge, about three-fourths the length of 

 the tergal border, and a moderate articular furrow. There is no ad- 



Fn;. 72. BALANUS TANTILLUS. a, LABRUM. I), 12TH SEGMENT OF CIRRUS vi. c, MANDIBLE. 



ductor ridge whatever. Pit for the lateral depressor muscle is small, 

 deep, and upon the basal margin. 



The tergum (pi. 48, figs. 2, 26) is flat externally, with very fine 

 growth-striae. No trace of a band or depression leading to the spur, 

 which is rather narrow and rounded distally, and passes gradually 

 into the basal margin. A basiscutal angle is hardly perceptible, the 

 spur forming a continuation of the scutal margin. Inside there is a 

 high but short articular ridge and a rather broad articular furrow. 

 Crests for the depressor muscle are strongly developed. 



Compartments. The walls are thick and massive for so small a 

 barnacle. Inside irregularly ribbed close to the thick base, smooth 

 above. Sheath short, not overhanging, though the wall is somewhat 

 hollowed below it. Radii wide, transversely grooved, with the sum- 

 mits oblique; edges thick and septate, as are the opposed sutural 

 edges. Ala3 wide. 



Basis rather thin, calcareous, poreless. 



Greatest diameter, 3.3 mm.; height, 1.6 mm. (largest example). 



Greatest diameter, 2.5 mm.; height, 1 mm. (usual size; type). 



