76 BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



The basis, in the small form, is flat, porous, often with an under- 

 lying multicellular layer. In the large form, the basis is lengthened, 

 forming half or three-fourths of the walls, and the cavity usually 

 tapers downward. According to Gravel, it consists of two portions, 

 the true basis and an intermediate segment which he terms the false 

 wall ("pseudo-muraille"), wherein the porous hypodermic layer is 

 reduced to a minimum, the greater part being solid and secreted by 

 the mantle. 



The littoral form of B. psittacus, which is what the Abbe Molina 

 described, is not a very large barnacle, in shape from conic to cylin- 

 dric. In a series from Valparaiso the diameter is 35 to 40 mm. ; height 

 40 to 45 mm. The basis is flat or but little built up. The walls are 

 thin or moderately thick; and the parietes are usually ribbed near 

 the summits, but sometimes part or all of them are nearly smooth. 

 A series of varying sizes, down to 2.5 mm. diameter shows that in the 

 early stage, up to about 6. mm. in diameter and height, the parietes 

 are quite smooth. In some individuals a ribbed stage then ensues, 

 to be followed by another smooth stage. In individuals 6 mm. in 

 diameter the opercular valves are exactly as in adults, except that 

 the furrow of the terguni is not closed. At this stage the radii are 

 very narrow. 



Some of the specimens from Arica show the best development of 

 the ribs (pi. 18, fig. 3). The inside, below the sheath, is strongly 

 ribbed. 



Examples from Chincha Islands, "abundant on the shore rocks," 

 and others from the Pescadores Islands, collected by Dr. R. E. Coker, 

 are small, though clearly adult. They measure 35 to 45 mm. high, 25 

 to 35 mm. in basal diameter. The shape is conic or vertical-sided; 

 parietes irregularly ribbed, radii rather wide. The orifice is ovate, 

 or angular at the sides and truncate at the rostral end. The color is 

 dull whitish gray, sometimes partly dull pink. The parietes are 

 ribbed lengthwise inside below the sheath. The basis is as flat as 

 circumstances permit, and either forms no part of the side walls or is 

 but slightly excavated. A group from the Chincha Islands, No. 38692 

 U.S.N.M., is figured 1 of the natural size in plate 16, figure 4, of my 

 paper on Peruvian barnacles. 



A very large and massive form of B. psittacus is found on the 

 Chilean coast. The lengthened basis forms about one-half of the 

 total height of the side walls. It is irregularly plicate longitudinally, 

 the folds rounded; compartments usually worn, and giving foothold 

 to numerous young of its own kind, B. Isevis nitidus and Verruca 

 Isevigata. Orifice nearly as large as the tube. They grow in groups, 

 diverging from the base. The largest in a group from Conception 



Proc. U. S. Nat. Mus., vol. 37. 



