PYRGOMA STOKESII. 361 



attached to the edge of the cup of a Caryophyllia, in deep water, but at 

 St. Jago within the tidal limits ; Mus. Brit., Cuming, Lowe, &c. 

 Fossil in the Coralline Crag, Ramsholt ; Mus. S. Wood. 



General Appearance. — Shell steeply conical, slightly compressed, 

 the lower part with rounded, approximate, radiating ribs : colour 

 dull purplish-red : orifice oval, small, and narrow. The basis is not 

 deeply conical, and occasionally is even flat. Generally it stands 

 exserted ; but in the Coralline Crag specimens, it is almost wholly 

 imbedded. Externally it is furnished with ribs corresponding with 

 those on the shell. The largest recent specimen which I have seen, 

 from St. Jago, was *22 of an inch in basal diameter ; but some few of 

 the British specimens are nearly as large, and one of the fossils from 

 the Coralline Crag a little larger. 



The Scuta and Terga are of the ordinary shape of these valves in 

 Balanus and its allies. Scuta triangular, with the basal margin a little 

 curved and protuberant : adductor and articular ridges distinct from 

 each other, moderately prominent : there is a small hollow for the 

 lateral depressor muscle. Terga triangular, with the spur rather 

 narrow, moderately long, placed near, but not confluent with the basi- 

 scutal angle of the valve. The basal margin forms an angle rather 

 above a right angle with the spur. Internally, the articular ridge and 

 crests for the depressor muscles, feebly developed. 



Internal Structure of the Shell and Basis. — Internally, the shell is 

 ribbed more or less prominently. The lower edge of the sheath, which 

 is reddish, and extends far down the walls, seems always to project 

 freely. In several specimens there were on each side, at the carinal 

 end of the shell, a trace of a suture, which could be perceived only on 

 the sheath. The basis appears always to be permeated by minute tubes 

 or pores, though these are sometimes rather difficult to be seen. 



2. Pyrgoma Stokesii. PL 12, fig. 6. 



Megatrema Stokesii. J". E. Gray, (sine descript. aut figura) 



Anuals of Philosophy, (new series), 

 vol. 10, Aug. 1825. 



Shell moderately conical, pale-pur pits k red: orifice oval: 

 basis not permeated by pores, deeply imbedded in the coral: 

 scutum and tergum sub -triangular. 



Hab. — Imbedded in the Mycedia (Agaricia) agaricitcs ; therefore from the 

 West Indies;* Brit. Mus. and Stutehbury. 



* I am greatly indebted to Mr. Dana for having named for me the coral in which 

 this species was imbedded, and informing me that it is a West Indian species. 



