THE SESSILE BARNACLES. 53 



incidence of the immediate environment, and partly because many 

 characteristics are common to several of the local forms, each having 

 a different combination of characters, which, singly, are also common 

 to some others. Working with specimens in large part of uncertain 

 habitat, partly taken from ships, which had successively picked up 

 the forms of different ports, an appearance is given of intergradation 

 of characters greater than the actual condition. 



. The problem can be solved only when well-localized material can 

 be studied, and all parts of the organism compared. My material is 

 fairly competent only for America, and I have therefore not been 

 able to undertake a thorough revision. 



KEY TO SPECIES OF MEGABALANUS. 1 



a 1 . Apex of the tergum acute but not produced in a beak. 

 b l . Scuta strongly sculptured externally. 

 c l . Basal margin of the tergum forming nearly a straight line on both sidea of the 



spur; no distinct crests for the depressor muscle. 

 d l . Shell extremely massive, often elongated in the carino-rostral diameter. 



B. ajax, p. 74. 



d 2 . Shell usually strong but not massive, approaching circular in outline. 

 e 1 . Shell of moderate or large size ; basal breadth of scutum less than its height. 



B. tinlinnabulum, p. 54. 

 e 2 . Shell very small; basal breadth of scutum at least equal to the height; 



no adductor ridge B. algicola, p. 72. 



c 1 . Basal margin between spur and basiscutal angle concave, forming an arc. 

 d l . Spur placed half its own width from the basiscutal angle; inner lamina of 



parietes normal. 

 e 1 . Pale rose; basal border of scutum somewhat concave. . .B. decorus, p. 77. 



e 2 . Whitish; basal border of the scutum convex B. campbelli. 



d 2 . Shell purplish dark brown ; inner lamina of the parietes cellular; spur placed 



about H times its own width from the basiscutal angle B. vinaceus. 



b 2 . Scuta marked externally with delicate growth-striae only, and without an adduc- 

 tor ridge. Shell pale rose, with deeply toothed orifice B. tulipiformis. 



a 2 . Apex of the tergum produced in a long beak. 



6 1 . Adductor ridge of the scutum united with the articular ridge, inclosing a cavity 



which extends nearly to the apex of the valve. 

 c 1 . Beak of the tergum purplish; spur placed at less than its own width from the 



basiscutal angle B. psittacus, p. 75. 



c 2 . Beak of the tergum white; spur removed from the basiscutal angle by at least 



its own width B. maxillaris, p. 77. 



I 2 . Adductor ridge not united with the articular ridge to form a cavity. Shell 

 usually purplish black B. nigrcscens. 



The following species of Megabalanus are not represented in the 

 United States National Museum. 2 



B. tulipiformis Darwin. Mediterranean, Madeira. 

 B. nigrescens Lamarck. Australia. 



Taken with slight alteration from Gruvel's Monographic des Cirrhipedes. 



2 Specimens of tulipiformis and nigrcscens are contained in the collection of the Academy of Natural 

 Sciences, leaving two species which I have been unable to study, both of them, I believe, known from the 

 original lots only. The specimens of B. decorus in the United States National Museum have no opercular 

 valves. 



4729 Bull. 9316 5 



