30 I BALANTD.E. 



A. spongites and fenestrata it is furrowed in the line of the 

 spur. The articular ridge and furrow are well developed in 

 A. fat est rata and purpurata. The crests for the tergal 

 depressor muscles are either absent or very feebly developed. 



Structure of the Parietes and Radii. — The parietes are 

 not porose ; internally, they are either smooth, or slightly, 

 or strongly ribbed in longitudinal lines ; the presence of 

 these ribs, which are homologous with the parietal septa 

 in Balanus, is variable even in the same species. In 

 A. sporillus the inner surface is curiously reticulated. The 

 sutural edges of the radii are either smooth, or very slightly 

 crenated by the septa, in lines parallel to the basis. The 

 upper margins of both radii and alse are always more or less 

 oblique. The radii sometimes do not extend down to the 

 basis ; and in this case, as will presently be described, 

 apertures are left in the lower half, between the compart- 

 ments. In A. plans and laevigata the internal margin of the 

 wall of each compartment, from the sheath to the basis, 

 projects inwards, forming inside the shell as many double 

 ridges (PL 9, fig. 5 5), as there are compartments, namely, 

 six : a nearly analogous structure occurs in certain species 

 of Balanus. The basal edge of the sheath, in most of the 

 species (5 b, 9 5), depends freely, and is hollow beneath, but 

 this is always a variable point. 



Basis. — The base is either saucer or cup-shaped, but in 

 A. cyathus it is almost flat ; it is generally symmetrical and 

 smooth, with the lines of growth closely approximate. In 

 A. fenestrate the basis is commonly as deep, as the shell is 

 high. The edge, in several of the species, is crenated with 

 minute teeth or notches; and these are so large in some speci- 

 mens of A. sulcata and cyathus, as to make the edge almost 

 pectinated. In A. glans, and in a lesser degree in A. Icevigata, 

 there are six knob-like teeth (fig. 5 a), corresponding with 

 the points of junction, between the basal edges of the in- 

 wardly prominent margins of the six compartments, and 

 the basis : in those specimens, in which the six teeth are 

 largely developed, six ridges produced by their successive 

 development, extend down towards the centre of the basal 

 cup. When the basal cup is dissolved in acid, there is 

 left a little animalised tissue and an external membrane, 

 formed as usual in slips, and furnished with blunt 



