(sect, d), balanus crenatus. 263 



i 



muscle is small, but variable. The terga are rather small : the spur is 

 short, and placed at rather less than its own width from the basi- 

 scutal angle ; the basal margin slopes a little towards the spur, of 

 which the lower end is rounded or bluntly pointed in a variable degree. 

 There is no longitudinal furrow, hardly even a depression. Internally, 

 the articular ridge is very prominent in the upper part ; the crests for 

 the tergal depressores are well developed, but variable. 



Compartments. — The internal carinal margin of each compartment, 

 from the sheath to the basis, generally, but not invariably, projects a 

 little inwards beyond the general internal surface of the shell, in a 

 manner not common with the other species of the genus : the basal 

 edge of this projecting margin rests on the calcareous basis, and is ere- 

 nated like the basal edges of the longitudinal parietal septa. The 

 whole internal surface of the shell is ribbed, but the ribs are not very 

 prominent. The parietal tubes are large, and are crossed in the upper 

 part, and often low down, by transverse thin septa : the longitudinal 

 parietal septa are only slightly denticulated at their bases; occasionally 

 they divide at the basis close to the outer lamina of the parietes, 

 making some short outer subordinate pores. In the circular furrow 

 beneath the lower edge of the sheath, there are sometimes little ridges, 

 dividing it into small cells : sometimes, however, this furrow is rilled 

 up by irregular knobs of calcareous matter. The radii are always 

 rather narrow, and often they form mere linear ribbons of nearly uni- 

 form width along the edges of the compartments. Their summits or 

 edges are always more or less irregular and jagged : they form an 

 angle with the horizon of generally above 40°. Their septa are fine, 

 and barely or not at all denticulated. The alae have oblique summits: 

 their suteral edges are rather thick and distinctly crenated. Basis- 

 flat, calcareous, very thin, with the surface slightly marked by ra- 

 diating furrows, which furrows answer to the radiating pores that occur 

 in the bases of most species. In a club-shaped arctic specimen, one 

 inch and a half in length, the summit being half an inch and the base 

 only one fifth of an inch in diameter, the basis was still calcareous, 

 thick, and not permeated by pores. 



Mouth: lab rum with six teeth: mandibles with the fourth tooth 

 minute or rudimentary, and the fifth generally confluent with the infe- 

 rior angle. Maxillse with generally, but not invariably, a small notch 

 under the upper pair of great spines. Cirri, first pair with the rami 

 very unequal in length, one ramus being nearly twice the length of the 

 other; in a large specimen having .a cylindrical shell the proportional 

 numbers of the segments in the two rami of the first cirrus were ten to 

 twenty-three ; in a small conical specimen the numbers were only eight 

 to thirteen. The second cirrus has only two or three more segments than 

 the shorter ramus of the first pair: the third cirrus has one or two more 

 segments than the second ; but it is nevertheless decidedly longer than 

 the second. On the dorsal surfaces of both segments of the pedicel of 

 the third cirrus, there is a tuft of fine spines. The segments of these 

 three pairs of cirri are not much protuberant in front. The segments 

 of the posterior cirri have, each, four, or five, or six pairs of spines. 

 Penis, with a straight, sharp, short point on the dorsal basis. 



