(SECT. A), BALANUS TrNTINNABULUM. 199 



one specimen of var. communis, and it is generally prominent in var. 

 coccopoma (1/), and most remarkably so in var. concinnus (Iff). The 

 cavity for the lateral depressor muscle is generally very slight ; but in 

 the two vars. just mentioned, and sometimes in var. communis, it is 

 deep. In var. concinnus (Iff) there is a remarkable plate developed 

 for the attachment of the rostral depressor muscle. The scuta are 

 coloured either dull-purple or reddish, or striped longitudinally white 

 and blue. The surface is sometimes externally depressed in the line 

 of the adductor ridge ; and in young specimens there is sometimes, 

 along this line, a chain of pits (1 h), as in full-grown specimens of B. 

 trigonus and Icevis. 



The Tergum (PI. 2, figs. 1 i to 1 o) is broad, with a generally closed 

 longitudinal furrow ; this furrow is open in young specimens, and it is 

 often, but not always, open in rather large specimens of var. occator; 

 it is always open in var. d'Orbiffnii (PL 2, fig. 1 m), and sometimes in 

 var. concinnus. Apex barely beaked, except in var. spinosus, in which 

 it is sensibly produced. Spur placed either very nearly in the middle 

 of the basal margin, or when least medial, it stand" at above its own 

 width from the basi-seutal angle ; yet in some specimens of var. occator 

 the spur is less than its own width from this angle. The basal margin 

 (1 i), on opposite sides of the spur, either forms a nearly straight line, 

 or the scutal portion descends lower than the carinal portion, and 

 curves very regularly towards the spur; this is the case in var. 

 oVOrbiffnii (1 n), and in some specimens of var. occator. The carinal 

 half of the basal margin generally forms an angle with the spur of only 

 a little above a rectangle. The spur varies a little in length and 

 breadth, but never exceeds one fourth of the greatest breadth of the 

 valve. The scutal margin is broadly inflected, the inflected portion 

 forming either a right angle, or somewhat less than a right angle, with 

 the exterior surface of the valve. Internally the articular ridge is 

 prominent, and is either considerably or slightly curved ; it extends 

 down either about half, or three fourths, of the length of the valve. 

 The spur is produced for a considerable distance up the internal sur- 

 face of the valve as a prominence. The crests for the tergal depressor 

 muscle are very feeble. In one specimen, in which both the shell and 

 operculum had undergone much disintegration, the scuta and terga 

 were calcified together. 



Compartments : their exterior appearance has been already described. 

 The parietal tubes are not large ; they are generally crossed by trans- 

 verse septa in their uppermost part ^ but they are sometimes almost 

 solidly filled up by dark shelly layers. The internal surface of the 

 parietes is more or less plainly ribbed ; in old specimens, however, it 

 generally becomes smooth. The radii have their septa denticulated on 

 both sides ; and they are porose, that is, the interspaces between the 

 septa are not filled up solidly. The radii generally extend from tip to 

 tip of the adjoining compartments, that is, their summits are parallel 

 to the basis ; but in three not common varieties, viz., vars. intermedins, 

 occator, and d' ' Orbignii (the former of which at least must, without the 

 smallest doubt, be ranked as a mere variety), their summits are oblique. 

 I have occasionally met with specimens of var. communis with oblique 



