(sect, c), balanus 1MPR0VISUS. 251 



prominent, but little reflexed : adductor ridge straight and very pro- 

 minent, varying a little in its distance from the articular ridge ; there 

 is scarcely any depression for the lateral depressor muscle ; the upper 

 internal surface of the valve is roughened with ridges. Terga, with a 

 moderately deep longitudinal furrow ; spur short, rather narrow, with 

 the end rounded, placed at less than its own width from the basi-scutal 

 angle ; in the Rio Plata specimens the spur is close to this angle : the 

 basal margin is generally straight on opposite sides of the spur, but 

 sometimes on the carinal side it is a little hollowed out. The lines of 

 growth are upturned along the carinal margin, which consequently is 

 a little protuberant, but to a varying degree. The crests for the de- 

 pressores are extremely distinct and prominent. In the varieties having 

 the basi-carinal margin hollowed out, and the carinal margin protube- 

 rant, there is a marked resemblance to the peculiar tergum of B. 

 eburneus. 



Walls: the parietal pores are tolerably large, and are crossed by nu- 

 merous transverse septa : the longitudinal septa are very finely denti- 

 culated at their bases, but occasionally almost smooth. The radii are, 

 as stated, extremely narrow, and very remarkable from their smooth 

 rounded edges ; their septa are barely denticulated. The alee are re- 

 markably protuberant ; they have their summits much less oblique than 

 those of the radii, and sometimes they are almost parallel to the basis : 

 their sutural edges are coarsely crenated. Basis, flat, thin, permeated 

 by pores, but the pores do not generally run to the very centre ; they 

 are, as usual, crossed by transverse septa. 



Mouth: the labrum is the most remarkable part; on each side of 

 the central notch there are generally two teeth ; and on the two sides 

 of the notch itself nine or eleven smaller teeth, decreasing regularly 

 in size downwards till they become so minute as to be hardly visible 

 even under the compound microscope ; thus, in the two specimens 

 closely examined, there were altogether twenty-two and twenty-six 

 teeth on the labrum. Mandibles with the two inferior teeth reduced to 

 mere knobs : maxillae with the lower part of the edge bearing two large 

 spines, and generally, but not always, forming a step-formed projection. 

 Cirri: the ramii of the first pair are but slightly unequal; in one 

 specimen examined there were fifteen segments in one ramus and 

 twelve in the other : segments very protuberant in front. Second 

 cirrus with the segments only slightly protuberant ; segments thirteen. 

 Third cirrus longer than the second pair, with the rami rather unequal 

 in length : there is a tuft of long spines on the basal segment of the 

 pedicel of this cirrus. Fourth cirrus twenty-two segments. Sixth 

 cirrus, in the same individual, thirty-four segments : on each of these 

 segments there are five or six pairs of spines. I may specify that the 

 longer ramus of the first cirrus of a large Rio Plata specimen had 

 twenty-four segments. 



Varieties, affinities. — When I first met with the var. assitnilis, mis- 

 guided by its general aspect, I did not doubt that it was specifically 

 distinct ; I was strengthened in this view by the absolute identity of 

 several hundred specimens attached to two vessels from Jamaica and 

 Trinidad, in the West Indies, with one specimen from Charlestown, in 



