THE SESSILE BAKXACLES. 



187 



59d). Dfirwin notes that specimens he exammed had 7, 8, and even 

 10 pairs of spines. The lower two points of the mandible are longer 

 than usual, but I noticed no other differences in the cirri and mouth 

 parts. 



Darwin found various of these characters in other combinations 

 with characters of normal B. halanokles, in some European indi- 

 viduals, and therefore did not segregate the. form as a new species. 

 Mv experience with the variety is too limited to enable me to form 

 an opinion as to its status. It does not seem to be a form due to any 

 direct reaction to environment, as it occurs in the immediate prox- 

 imity of normal B. halanoides. 



Alaskan forms of B. halanoides. — Specimens from Unalaska, Cat. 

 No. 5S0G3, growing on Balanus cariosus, with Chtlmmalus dalli, are 

 small, diameter 5 to 6 mm., rugged, resembling plate -io, figure ic, 

 or with more ribs. The tergal and basal margins of the scutum are 

 of equal length, and the valve is curved between apex and base, con- 

 cave externally, as in B. crenutus curviscutum-. The tergum has a 

 peculiarly concave scutal margin, very broadly inflected below. Free 

 apex is unusually long. The spur is shorter than normal for the 

 species. The apices of the opercular valves diverge somewhat Avhen 

 in the occluded position. 



The labrum has three strong teeth on either side of the median 

 notch. Maxilla has a very small notch, in which several small spines 

 stand, below the two upper spines, and a rather prominent pair of 

 spines near the lower angle. 



The first cirrus has 15 and 9 segments, the posterior ramus three- 

 fourths as long as the anterior. Second cirrus with subequal rami of 

 10 segments. Third cirrus of 10 and 9 segments, the posterior 

 ramus three-fourths as long as the anterior, and without spinules on 

 the segments. The later cirri have segments with, at most, 7 pairs 

 of spines. 



This is the only West American lot referable to B. halanoides 

 which I have seen, and here the terga and maxillte difl'er somewhat 

 from all Atlantic specimens examined. Other Alaskan forms seen 

 may be segregated as a subspecies. 



ErROPEAX. 



