10 INTRODUCTION. 
(r, fig. 2, 3), they are wedge-formed, with their points downwards; their summits (and 
this is often a useful specific character) are either parallel to the basis, or as in fig. 1, 2, 
oblique. The radii are sometimes not developed.* 
The ale (so called by Dr. Gray) are overlapped by the radii, and by part of the walls ; 
they usually extend only about half way down the compartment (a, fig. 3, and 4); their 
summits are either parallel to the basis or oblique. The ale of the several compartments, 
together with the internal, upper, thickened surfaces of the walls, against a shoulder of 
which the sutural edges of the alee abut, have been called (by Dr. Gray) the sheath (vagina). 
The upper and greater portion of the sheath is marked by transverse lines, caused by the 
exuviation of the opercular membrane, as that membrane may be called, which unites the 
operculum all round to the sheath, or upper internal surface of the shell. 
The carina has always two ale, as in fig. 4. 
The carino-lateral and lateral compartments have always an ala on one (the rostral) 
side, and a radius on the other (the carinal) side, as in fig. 3. 
The rostro-lateral compartment (not at present known to occur in any fossil) has always 
radii on both sides, as in fig. 2. 
The rostrum has normally ale on both sides, as in fig. 4; but in many recent, and 
all the fossil species yet known, it has radii on both sides, as in fig. 2, owing to its fusion 
with the rostro-lateral compartments on both sides. 
The walls of the shell, the basis, and the radii, are in very 
mmany cases composed of an outer and inner lamina, united 

together by septa; a set of pores or tubes being thus formed. 
Basal edge of wall of compartment ‘The points of the septa generally project beyond the lamine, 
in Balanus tintinnahbulum, a, a, outer 
SH a tenia naling tiginnsrang | od Bre denticulated on both sides, as shown in the accompany- 
outer laminz, with their ends denti- 
culated. ing woodcut. 
Onerculum, or opercular valves.—These consist of a pair of scuta and a pair of terga. 
They are joined to the sheath of the shell by the opercular membrane. 
Scutum (fig. 5): this valve is generally sub-triangular, and its three margins are 
the dasa, the dergal, so called from being articulated with the tergum, and the oceludent, 
so called from opening and shutting against the opposed valve. ‘The angles are named, 
from the adjoining margins, as dasi-tergal, &c.; the upper angle being the apex. The 
scutum is ordinarily articulated to the tergum by an articular ridge (crista articularis), 
running up to the apex of the valve, and by an articular furrow, which latter receives the 
' The radii have been called by Ranzani and De Blainville “‘areze depresse’’ (the parietal portions of 
the compartments being the “‘aree prominentes’’); by Poli, “areze interject ;’ by Gray, ‘‘sutures ;” 
by Coldstream, “compartments of the second order,” (the parietal portions being those of the first order) ; 
by some authors, ‘‘intersticia.’’ I may here add that the scuta are the “‘ventral valves” of Gray, the 
“anterior” of Ranzani, and the “‘ inferior opercular” of De Blainville: the terga are the “ posterior valves’’ 
of Gray and Ranzani, but the “ superior opercular” of De Blainville: the rostrum, on the other hand, is the 
“anterior valve” of Ferussac, and the ‘ventral’? of De Blainville; the carina being the “dorsal valve’’ of 
the latter author. 
