POLYTREMA. De Ferussac. Dict. Classique d’Histoire Naturelle, 1822.
LEPAS. Gmelin. Systema Naturæ, 1789.
BALANUS. Bruguière. Encyclop. Method., 1789.
------ Lamarck. Animaux sans Vertèbres, 1818.
[105] From a note by the Editor, it appears that Schumacher’s essay appeared before the number of the Journal containing Leach’s paper, so that Schumacher’s name must be adopted.
[106] I have not seen a complete copy of this work, and give the title from a catalogue; the running heading of the part containing the Cirripedia, is “Opuscoli Scientifici.”
Compartments four, sometimes externally calcified together: parietes permeated by pores, generally forming several rows. Basis flat, irregular, calcareous, or membranous.
Hab.—Throughout the tropical and warmer temperate seas.
General Appearance.—The shell is conical, more or less depressed, and very rarely, even when growing in crowded groups, becomes cylindrical or elongated. The orifice is seldom large, generally diamond-shaped or oval. The colour is either nearly white or purple, occasionally even inky black, or very dark green, and sometimes of a pale pink peach-blossom. The surface is sometimes smooth, but more commonly longitudinally ribbed; the outer lamina of shell is very often wholly corroded away, excepting close to the basis, leaving the solidly upfilled parietal tubes exposed: these give the shell a striated appearance, or they appear like flattened tapering points adpressed to its surface (Pl. [10], fig. [1 b]): Lamarck attempted to express this appearance, by using the specific name of stalactiferus. The colour of the shell depends, to a considerable extent, on the colour of the shelly matter in these exposed parietal tubes. We shall presently see that the corrosion and disintegration, to which some of the species are so liable, plays an important part during their growth. The radii are either well developed, as in most of the species; or they are entirely absent, as in the great majority of specimens of [T. porosa] and [serrata]. In many individuals of [T. porosa] and [purpurascens] not only are the radii absent, but the four compartments are calcified together without any trace, on the external surface, of the sutures. The largest specimen which I have seen of [T. porosa], which is the largest species, was two inches in basal diameter, and nearly one inch and a half in height.
Scuta.—These valves are sub-triangular, and either longitudinally or transversely elongated. Externally, the growth-ridges are moderately prominent, and in [T. costata] and [cœrulescens] they are crossed by longitudinal striæ. Along the occludent margin, the inflected extremity of each alternate growth-ridge is generally much thickened,—a set of teeth being thus formed, by which the two valves are locked together. In [T. porosa], this character is variable, for sometimes every alternate ridge, and sometimes only two or three ridges, separated from each other by several growth-ridges, are thus developed into teeth. The articular ridge is either moderately prominent, or is extremely prominent, as in [T. cœrulescens]; but the lower edge in no case depends as a free, hinge-like style, as sometimes in [Balanus]. The adductor ridge is generally well developed and distinct from the articular ridge: in [T. purpurascens] it is very blunt: in [T. serrata] it is united to the articular ridge half way up it, thus forming a deep tubular cavity running up to the apex of the valve: in [T. cœrulescens], the adductor ridge is very short, and is united to, or almost continuous with, the lower end of the articular ridge, a small sub-cylindrical tubular cavity being thus formed. Small crests exist for the attachment of the rostral and lateral depressor muscles, in most of the species, excepting [T. purpurascens] and [costata], in which, however, more especially in the former, there are, instead of crests, minute pits for the attachment. These crests vary much in prominence in the same species.