GENUS PYRGOMA. 355 



I feel no hesitation in including the above several genera 

 in one genus. In external appearance the P. monticularice 

 (PL 13, fig. 5 a), which forms the genus Daracia of Gray, is 

 the most distinct, but it is so intimately allied to the two, 

 indeed to the three, foregoing species, that it cannot be 

 separated from them. Of the first five species, P. grande, con- 

 jugatum, and cancellatum, form a graduated series, but with 

 the steps very distinct, the chief difference being in the length 

 of the spur of the tergum ; for the fact of the scutum and ter- 

 gum being calcified together in P. grande and conjugatum, 

 and distinct in P. cancellatum, is certainly unimportant, as 

 may be inferred from what we shall see in comparing together 

 the last four species of the genus, and from what we shall see 

 in Creusia spinulosa. The three above-mentioned graduated 

 species are connected with the last four species of the genus, 

 by several points of resemblance between P. grande and 

 crenatum. The first two species, namely, P. Anglicum and 

 Stokesii, are the most closely related together, and may in- 

 deed possibly be identical ; these two, in all the characters 

 derived from the opercular valves, resemble Balanns and 

 other ordinary forms, and for this very reason they have 

 some claims to be genetically separated from the other 

 species of Pyrgoma ; for in these latter, the. opercular valves 

 seem to have broken loose from all law, presenting a greater 

 diversity in character than do all the other species of Bala- 

 ninse and Chthamalinse taken together. 



General Appearance. — The shell consists of a single 

 piece, generally without any suture, even on the internal 

 surface; and this is the case, at least in P. Anglicum, in 

 extremely young colourless specimens : nevertheless, in 

 some specimens of this very species, and of P. conjugatum, 

 there were traces of two, but only two, sutures on the 

 sheath, one on each side towards its carinal end. The shell 

 is much depressed or actually flat ; and I have seen speci- 

 mens even slightly concave ; in P. Anglicum, however, the 

 shell is steeply conical. The outline is generally oval; 

 but in P. monticularice it is extremelv irregular. The 

 surface is generally furnished with more or less prominent 

 ridges, radiating from the orifice, which is oval and small ; 

 sometimes, as in P. monticularice, excessively small. The 



