THE SESSILE BARNACLES. 153 



The first and last of these differences are the most definite, and in 

 my experience they are invariable characters in each species. B. 

 rostratus is rarely ribbed, and never ribbed so strongly as the common 

 forms of B. lalanus. In the latter species the inflexed scutal border 

 of the tergum terminates basad in a lobe, while in B. rostratus it is 

 straight or obliquely cut off, and the spur stands nearer the basiscutal 

 angle. 



B. balanus, like its Pacific relative B. rostratus, is far from being a 

 homogeneous species. In going over large numbers from several 

 regions one gains the idea that there are several subordinate races. 

 Differences in the sculpture, the structure of the walls, the dentation 

 of the labrum, and the number of spines on the cirri are readily 

 found. Yet, without making a great number of dissections and de- 

 voting several weeks or perhaps months to the problem, I do not 

 feel competent to distinguish between fluctuating variation, variation 

 due to the effect of station on the individual, on the one hand, and 

 heritable variation of racial import on the other. It would be an 

 ill service to science to attempt the diagnosis of local races without 

 such an investigation as I have indicated. The results of my work 

 so far as I have gone, are briefly given below. 



Station. B. balanus usually makes its home on shells, probably 

 because they are the most abundant solid objects available in the 

 depths it inhabits. When growing on objects with an even surface, 

 such as Pecten magellanicus or Modiolus modiolus, they reach their 

 ideal development ; but on the banks they are also common on Buc- 

 cinum, Tritonifusus, Aporrhais, T erebratulina, on pebbles, or more 

 rarely growing on one another. B. ~balanus is frequently associated 

 with B. ha>nieri and B. crenatus. It is rather common in from 20 to 

 90 fathoms, but seems to drop out of the fauna rather abruptly at 

 the latter depth, as it would surely have been taken if present at some 

 of the very numerous Albatross stations in deeper water. South of 

 Cape Cod it seems exceptional above 18 or 20 fathoms, where B. 

 eburneus and B. amphitrite niveus are the prevalent species. Far- 

 ther north, on the Maine coast and northward, B. balanus is some- 

 times taken at depths of only a few fathoms. On Coffin Island, Mag- 

 dalen Islands, in the Gulf of St. Lawrence, Mr. Bayard Long found 

 it at low water, on Mytilus edulis. 



Variations in the structure of the wall. The figures on plate 35 

 show the chief variations in the basal septa of the walls. Figure 5, 

 from Georges Bank, is the normal and most frequent condition. 

 There are from one to four ribs on the inner lamina between the com- 

 plete septa; outer lamina has several or many very short lamellae 

 within the margin. The rostrum here figured has 11 tubes. 



Plate 35, figure 3, represents portions of two rostra from Eastport, 

 Maine; strongly ribbed form. Here the inner and outer laminae of 



