THE SESSILE BARNACLES. 83 



barnacle for any other. Young specimens in which the sculpture of 

 the scutum and the emargination of the base of the tergurn have not 

 been developed, are perhaps most easily recognized by examination of 

 the cirri, which differ from those of improvisus, amphitrite and crenatus, 

 species inhabiting the same waters. 



B. eburneus and B. improvisus never develop ribs, and are never 

 colored. 



B. eburneus has some superficial resemblance to small specimens 

 of B. Jiameri. It is closely related to B. improvisus, and small or 

 young individuals in which the tergum has not assumed the bird's-foot 

 shape, and the stria? of the scutum have not yet appeared, are often 

 extremely hard to distinguish from that species. Darwin writes: 



I for some time mistook the var. assimilis of B. improvisus for the young of B. 

 eburneus. But I found in the latter that the rami of the first pair of cirri are always, 

 even in the earliest youth, more unequal in length, and that each segment of the 

 posterior cirri bears a greater number of pairs of spines, there being, even in very 

 minute specimens, seven pairs. Moreover, after having examined scores of speci- 

 mens, I found I could almost always distinguish the two species by the smoothness 

 and curvature of the summits of the radii of B. improvisus; I entertain no doubt 

 whatever about the distinctness of the two species; indeed, when both are mature, 

 besides the greater size, striated scuta, etc., of B. eburneus, their general aspect is veiy 

 different. 



B. eburneus varies but little. The basal margin of the scutum is 

 not always so hollowed out as in the individual figured ; and of course 

 in quite young individuals it is not at all hollowed out. Pleistocene 

 examples from the Panama Canal Zone are quite similar to recent 

 ones from Massachusetts. 



B. eburneus often lives in brackish water. I found small ones on 

 the piles at Betterton, near the head of Chesapeake Bay, where the 

 water is but slightly brackish, the fresh-water snails Goniobasis and 

 Amnicola living in it. "Professor Wyman found it living about 50 

 miles up the St. Johns River, Florida, where the water was fresh 

 enough to drink, and the specimens lived well when transferred to a 

 vessel of perfectly fresh water." 



B. eburneus often attaches to ships' bottoms. No doubt the ex- 

 amples in the Jeffreys collection, taken by A. d'Orbigny at Rochelle, 

 were so transported. It has been reported by Herr Weltner from 

 Venice and Manila, but these records must have been based upon 

 ship-carried individuals. 



It does not often form crowded or superposed masses, as many 

 species do, and is far of tener found on wood and oyster shells than on 

 rocks. Mr. Win. J. Fox gave me examples taken from the screw of 

 his launch, in frequent use. 



Dr. Benjamin Sharp took specimens 23 mm. diameter from a 

 Nan tucket boat which had been in the water 98 days, from June 13 

 to September 21. In a lot in the Museum, from a boat which had 



