PYRGOMA CRENATUM. 371 



depressed spur : scutum and tergum furnished with a wide 

 occludent ledge. 



Hah. — Philippine Archipelago ; Singapore ; Mus. Brit., Cutning, Stutchbury ; 

 sometimes associated with Creusia spinulosa. 



Appearance and Structure of Shell. — Shell not distinguishable from 

 that of P. dentatum ; nearly flat, oval, white, sometimes pale pink, with 

 rather distant prominent ridges radiating from the moderately large 

 oval orifice. Shell solid, or permeated near the outer lamina by short 

 imperfect pores: internal surface smooth. The sheath descends nearly 

 to the base of the walls ; it is but little conspicuous, and its lower 

 edge is closely attached to the inner surface of the shell. Basis deep. 

 Diameter of largest specimen under '3 of an inch. 



Scuta : in this species the scutum is more abnormal than in any 

 other Cirripede in the whole family : this is owing both to the ad- 

 ductor ridge descending far beneath the basal margin, and to the great 

 development of the occludent ledge ; hence the middle and very narrow 

 portion of the valve alone answers to the scutum, as seen in other 

 genera. The whole valve, including the adductor ridge and the occlu- 

 dent ledge, is narrow, being more than twice as broad as high; but the 

 proportional width varies, owing chiefly (as in the last species) to the 

 varying width of the occludent ledge. This ledge commences a little 

 way from the rostral point of the valve, and gradually widening, extends 

 to the apex, where it is either as high or twice as high as the rest of 

 the valve. It is articulated bv a convex surface, and bv a hollow on its 

 under side with the occludent ledge of tergum. The scutum cannot 

 be said to have any tergal margin ; without, indeed, the articular 

 surface of the occludent ledge be thus called. The basal margin is 

 curved, and considerably reflexed, of which peculiarity we have seen 

 a vestige in the last species : the reflexion is not well shown, owing to 

 the foreshortening of the reflexed edge, in fig. 4 a : this reflexed edge 

 not being shown, causes the lines of growth to appear as if they ran 

 more transversely to the longer axis of the valve, than they really do ; 

 for they run nearly as in the scutum (fig. 3 e) of P. dentatum. The 

 direction of these lines of growth is of importance, for they show that 

 the elongation of the scuta is due to an inordinate increase in their 

 breadth, as compared to the same valves in ordinary species. The 

 adductor ridge, having a sinuous margin, runs from near the apex to 

 near the rostral angle: it descends below the basal margin about as far 

 as the height of the true valve, excluding the occludent ledge. Of 

 course this adductor ridge or plate lies beueath the membrane con- 

 necting the opercular valves with the sheath, and is concealed by it, 

 as long as these valves remain within the shell. The edge of the oc- 

 cludent ledge is straight, but the edge of the middle portion of the 

 valve, that is of the true valve, is much bowed. 



Terga (fig. 4 b) : these are of so irregular a shape that they can hardly 

 be described ; they may, however, be said to consist of two portions 

 joined together, of which the lower portion is a little elongated trans- 

 versely, of somewhat variable shape, with part of its surface considerably 



