308 BALANIDiE. 



orifice in the Balanidse. The walls are thick, and are formed of large 

 square tuhes. The internal surface of the shell is smooth. The sheath 

 (fig. 2 b) is much more elongated than the shell, for at the rostral end 

 it extends to the basal margin, and at the carinal end nearly to it, 

 whereas on the two sides it is some way distant from the base. The 

 orifice is considerably out of the centre of the sheath, being placed 

 nearest to the carinal end. The sheath has its lower edge slightly 

 prominent or free ; the lines of growth are neither turned up nor down 

 on the two sides, as in the last three species. When nearly full grown 

 the sheath is coloured dark purple, but when young it is white, hence 

 the upper part is white, surrounded by an oblong purple ring, and this 

 is surrounded by the white shell. The basal cup is deep, and internally 

 nearly smooth. The largest specimens were *3 of an inch in their longer 

 diameter. Great masses of the Millepora are absolutely studded with 

 this Pyrgoma, with usually more specimens on one side of the plate or 

 branch than on the other. They stand in approximately parallel posi- 

 tions, the broad or carinal ends of the orifices pointing upwards. 



Scuta. — The scuta and terga are closely united, and are often (per- 

 haps always) in some slight, though variable degree, calcified together ; 

 and hence they often break, rather than separate, at the line of articu- 

 lation. The external fissure or line of junction between them (fig. 2 c) 

 is oblique to the longer axis of the scutum ; in the uppermost part of 

 the valve it is sometimes almost obliterated. The two valves together are 

 nearly as long as the sheath, and consequently much longer than the 

 orifice of the shell. The scutum is much elongated, being fully four 

 times as broad as high. The valve narrows towards the rostral end, but 

 in a variable degree : the basal margin is hollowed out a little (but to 

 a variable amount), close to the basi-tergal corner. Along the occludent 

 margin a slip of the valve, widening downwards, is a little bent in- 

 wards, and this inflected portion is separated from the rest of the sur- 

 face by a slightly angular ridge, running from the apex to near the ros- 

 tral angle. Internally, at the basi-tergal corner, a slight ridge, parallel 

 and close to the basal margin (and which can be seen only when the 

 basal edge of the valve is held upwards), represents the adductor ridge, 

 which we have seen so largely developed in the last three species, and 

 shall again see in P. crenatum. The articular ridge (fig. 2 d) is extremely 

 prominent, consisting of a more or less rectangular shoulder. 



Terga : these are rather small compared to the scuta : they are 

 triangular and much arched : there is no trace of a spur. Internally 

 (fig. 2/), the articular ridge is central : there are some vestiges of crests 

 for the depressor muscle. 



Affinities. — This well-marked species, in the tendency of the oper- 

 cular valves to be soldered together, and in the remarkable absence of 

 a trace of a spur to the tergum, is allied to P. grande, but it is more 

 closely allied to the three following species. 



