THE SESSILE BARNACLES. 155 



There seem to be two chief races among the British specimens. 

 The typical form (pi. 33, figs. l-2, G-Gfr) is rather high with steep 

 walls, which have the ribs only moderately prominent and usually 

 irregular; radii wide. The external sculpture is readily modified 

 by the irregularities of the supporting object. 



The type-specimen x is represented in plate 33, figures 1, 1. The 

 surface is irregularly roughened by the laminae of the Pecten, and 

 the ribs are irregular. 



The other European race or "form" (pi. 33, figs 4, 5) is conic, 

 spreading and very rugged; rostrum with four strong ribs, laterals 

 and carina, three or four, carinolaterals with one strong rib; radii 

 rather narrow. Opercular valves as in typical B. balanus. The ribs 

 in this race seem very little affected by the irregularities of the sup- 

 port. Examples from Belfast Bay, growing on Pec-ten opercularis 

 (fig. 5), are similar to those growing on the smooth mussel Modiolus 

 modiolus (fig. 4). 



Donovan's Lcpas balanus is this form. His Lepas costata seems 

 to be the young stage of a similar form which I have seen from 

 Scarborough (fig. 4), and which differs only by having more ribs; 

 in a specimen 19 mm. in diameter there are seven ribs on the rostrum, 

 five on the lateral compartments. 



Northward B. balanus becomes larger than in England and Ire- 

 land. Darwin records a specimen 2.1 inches in diameter from the 

 Shetland Islands, and others equally large from glacial deposits of 

 the Isle of Bute. 



Arctic Ocean. The highest latitude for B. Ixdanus, or so far as I 

 know, for any cirripede, is Aberdare Channel, east of Alger Island, 

 latitude 80 21' 21" north, longitude 56 OS' east, in the Franz 

 Josef Land Archipelago. Here the Baldwin-Ziegler Expedition 

 obtained a series of rather peculiar specimens, Cat. No. 48195, 

 U.S.N.M., illustrated on plate 35, figures 1, la, 15. The form 

 is cylindric or barrel shaped, the orifice as large as the base. The 

 parietes usually show weak ribs in the upper part, and sometimes 

 they persist irregularly to the base. Radii very broad. Interior 

 strongly striate, the septa between outer and inner laminae about as 

 numerous as in the typical form, but with fewer interstitial points 

 than in typical Ixdanus. The scutum is thin, with even less trace 

 of an adductor ridge than usual. The tergum is remarkably nar- 

 row, its carinobasal angle rounded off, and the long, free apex is 

 not tinted (fig. 44). 



The mouth parts are normal, but the posterior cirri have an addi- 

 tional pair of spines. First cirrus with 23 and 10 segments, second 

 with 17 and 13. The third cirrus has many spinules on the segments 



1 1 owe the photographs of this typo to the courtesy of the authorities of the Linnean Society of London. 

 They were obtained by the kind intervention of Mr. T. H. Withers, of the British Museum. 



