372 balanid^:. 



depressed (compared with the rest of the valve), sometimes being even 

 concave : this concave portion apparently answers to the spur in other 

 cirripedes. At the scutal corner of the valve there is a shoulder (per- 

 haps answering to the inflected scutal margin in an ordinary tergum), 

 which locks into a hollow on the under side of the occludent ledge 

 of the scutum. The internal surface of the so-called spur is rounded 

 and convex. The upper part of the tergum is in main part formed 

 by a great occludent ledge ; but this, on its lower side, is bordered 

 by a narrow irregular slip, which, as shown by the lines of growth, 

 represents the whole of the ordinary valve, excepting, of course, the 

 spur already described. The occludent ledges of both valves support 

 some fine spines. 



Affinities. — Observing how extraordinarily the terga varied in P. den- 

 tation, and that the shells were identical in that and the present species, 

 it occurred to me at first that they might, perhaps, be both extreme 

 varieties of one form : but in the scutum of P. crenatum, the invariably 

 great development of the adductor plate, — the marked manner in which 

 its basal margin is reflexed, — the absence of a tooth-like articular pro- 

 jection, — and again in the tergum of P. crenatum, the invariably large 

 size of the concave spur, without any internal tooth, altogether con- 

 vince me that the two species n.ust be considered as distinct. This 

 species is allied to P. r/rande, in the scutum of that species having an 

 occludent ledge, though small, and a great adductor plate. I have only 

 further to remark, that the figure of the opercular valves, given in 

 Sowerby's Genera of Recent and Fossil Shells, is so good, that there 

 can not be the least doubt about the present identification. 



9. PYRGOMA MONTICULARI.E. PI. 13, fig. 5tf 5/. 



Pyrgoma (Dakacia) monticulaki^;. /. E. Gray. (!) Zoological 

 Miscellany, p. G, 1831. 



Shell of an irregular shape, with a roughened exterior 

 border; orifice minute, circular: scutum and tergum both 

 much elongated, calcified together without any suture, both 

 furnished with a broad occludent ledge. 



Hub. — Singapore ; Mus. Brit., Cuming, and Stutchbury. Sometimes asso- 

 ciated with Creusia spinulosa. 



Appearance and Structure of Shell. — Shell dull white, very irre- 

 gular in outline, sometimes rounded, more often unequally elongated, 

 and frequently star- shaped, — the projections being quite irregular. 

 Whole shell nearly flat, but with the central part saddle-backed, or 

 formed into a more or less prominent ridge, extending in the line of 

 the longitudinal axis of the animal's body: the circumferential portions 



