358 BALANIDJE. 



upwards on each side, at points corresponding with the line 

 of union between the scutum and tergurn, in a manner I have 

 not seen in any other cirripede ; and this sometimes gives the 

 appearance of two lateral sutures. I may here remark, that 

 the manner of growth in Pyrgoma is almost the converse of 

 that in Balanus, Tetraclita, and other allied genera ; for in 

 these latter, the basis increases in diameter, and the shell 

 chiefly in height ; whilst in Pyrgoma, the shell, from being 

 so flat, increases almost exclusively in diameter, whereas it 

 is the basal cup which is added to in height or depth. 



Basis. — This in all the species is more or less regularly 

 cup-formed or sub-cylindrical. In P. grande it penetrates 

 the coral to a surprising depth ; in P. monticularia it is 

 irregular in outline, corresponding with the shell. The 

 basis is generally almost wholly imbedded in the coral ; but 

 this is not the case with P. Ajiglicum, in which the basis is 

 generally exserted, as it is in a slight degree in P. grande. 

 The shelly layer forming the basis, in most of the species, is 

 very thin, and is finely plicated owing to its edge folding 

 between the ridges or septa that form the basal edges of the 

 shell ; this is very conspicuous in P. cancellatum. The basis 

 is not permeated by pores, except in P. Anglicum. In some 

 sessile cirripedes a cleft, covered only by membrane, may be 

 observed all round between the lower edge of the shell and 

 the basis ; a cleft of this nature is rather conspicuous in 

 P. crenatum, so that small portions of the septa on the in- 

 ternal surface of the walls can be seen from the outside. In 

 P. monticularice, an analogous structure, developed to an 

 extreme degree, presents a very different and unique appear- 

 ance ; the shell is nearly flat, and the smooth outer lamina 

 does not nearlv reach to the circumference, a wide border 

 being thus left exposed, which is roughened (PL 13, fig. 5 a, 

 5 c, 5 d, 5 e) by the exposure of the irregular septa. I have 

 not seen a fresh specimen, but there cannot be a doubt that 

 this border is properly covered by membrane. 



Animal's Bod//. — Prom some cause, perhaps from the 

 corals in which the species of Pyrgoma are imbedded, long 

 remaining damp, the internal organs are generally badly 

 preserved. I have received, in spirits of wine, only P. 

 Anglicum, but 1 have examined dry specimens, in a tolerable 



