184 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



solidly. In much worn individuals the tubular layer may be entirely 

 removed. The sheath is very short. Radii narroAv or almost obsolete. 



The basis is wholly membranous. 



Darwin further describes it as follows : 



Mouth : Lnbrum with the teeth on each side of the central notch un- 

 usually variable in number; I have seen specimens with only two on each 

 side, with four on each side, with five on one side and four on the other, 

 with five on one side and none on the other, and with six on both sides; 

 hence the total number ranges from four to twelve. Mandibles, with the 

 fourth and fifth teeth small, or quite rudimentary. Maxillae, Avith scarcely 

 even a trace of a notch under the upper pair of spines. Cirri : First pair, 

 with one ramus one-third or one-fourth longer than the other ; in one speci- 

 men the number of segments were 9 and 16 in the two rami ; second and 



Fig. 58. — Balanos balanoides. New Haven, c^labrum. 6^ maxilla, c, mandible. <Z, in- 

 termediate SEGMENT FROM CIRRUS VI. FiGCEES (I, h, d TROM A CONIC SPECIMEN, NO. 



32946. Fig. c from a tubular specimen. No. 32047, 



third rami short, very nearly equal in length, having in the first-mentioned 

 specimen respectively 10 and 11 segments. The sixth ciri'us in this same speci- 

 men had 25 segments, each segment being about as long as broad and support- 

 ing six pairs of spines. In the singular variety (a) the posterior cirri are 

 more elongated, and each segment supports 7 or 8, and in one case even 10 

 pairs of spines. The third pair is also in this variety proportionally rather 

 longer ; at the base of the third pair there is a tuft of fine spines. The penis 

 has not, as in B. crenatus, a point at its dorsal basis. 



The membranous basis amply distinguishes B. halanoides from 

 species of the region otherwise similar. When the barnacle is de- 

 tached, it may be known by the wall which is not ribbed within, thus 

 differing from B. crenatus. Conic, depressed forms have hot the 

 well-developed tubes and septa of most other species, and in most of 

 the lengthened, C3dindric, or club-shaped forms the tubes are mainly 

 closed at the basal edge. I believe that the scuta may be recognized 



