(sect, e), balanus balanoides. 269 



in the elongated varieties ; in these latter, there is oecasionally a mo- 

 derately deep longitudinal furrow : the spur is often exactly the same 

 shape as in B. crenatus, but it is apt to be rather longer (PL 7, fig. 2 c) 

 and more pointed : in var. (a) it is pointed (fig. 2 d ) in a very remark- 

 able manner. Internally, the articular ridge is decidedly more pro- 

 minent than in B. crenatus; the crests for the tergal depressor muscles 

 are either well developed or almost absent. From this description it will 

 be seen, how singularly the opercular valves of the common varieties of 

 these two species resemble each other. I may mention that in some 

 of the much elongated specimens, the muscles going to the opercular 

 valves partially lose their transverse strise, and become ligamentous. 



The Parietes are either quite solid, or more commonly are permeated 

 by minute pores, or by small irregular square tubes (PL 7, fig. 2 &), 

 which only run up each successive zone of growth, for very short dis- 

 tances, giving to the shell a cancellated structure, which from corrosion is 

 often externally visible. In the rather rare variety (a) the parietes are 

 permeated by regular tubes, extending up to the apices of the compart- 

 ments, but crossed by transverse septa. The longitudinal septa, when 

 such can be said to occur, in no case are denticulated at their bases. 

 The internal surface of the parietes is either quite smooth or is tra- 

 versed (PL 7, fig. 2 b) by very slight anastomosing ridges, but never, 

 even in var. (a), by regular longitudinal ribs, as in most other species. 

 The carinal margin of each compartment, on the inside, projects, as in 

 B. crenatus, inwards, beyond the general surface of the shell, and run- 

 ning down, rests on the basal membrane. The lower edge of the 

 sheath is rarely hollow beneath. The walls are lined by purplish, or 

 pale brown, or sometimes by almost black corium ; numerous tubuli 

 penetrate the under sides of the walls and opercular valves ; and it is 

 the intersection of these tubuli that gives the punctured appearance 

 to the often corroded surface of the shell. The radii are narrow, 

 generally very narrow ; they have their upper and outer margins, as 

 seen externally, very oblique, rounded and (when well preserved) 

 smooth ; their sutural edges are either quite smooth, or sometimes 

 just perceptibly pitted, like the basal margin of the walls, or occa- 

 sionally furnished with globular or arborescent little ridges. The 

 alee are also very oblique, but to a variable degree, sometimes only 

 slightly oblique : their sutural edges are either smooth or obscurely 

 crenated. Basis, membranous ; in some much elongated specimens, 

 during continued growth, the basal edges of the compartments approach 

 each other so closely as almost to touch, so that the whole shell be- 

 comes pointed at the bottom ; but on careful inspection I have never 

 failed to find, even in the most pointed specimens, a minute basal 

 membrane ; in other much elongated varieties, in which the shell has 

 apparently become too large for the animal's body, the basal mem- 

 brane, instead of being flat, becomes drawn up deeply inwards, so as to 

 touch the surface of attachment only close round the basal edges of 

 the shell. 



Mouth : labrum with the teeth on each side of the central notch 

 unusually variable in number ; I have seen specimens with only two 

 on each side, with four on each side, with five on one side and four on 



