PYRGOMA. 35 
in the external longitudinal ridges standing much further apart, and, consequently, in the 
furrows being much wider: each ridge is generally double. Although there is a good deal 
of variability in the character of these ridges in 4. wndulata, and likewise in A. spongites, | 
have not seen any form intermediate between them. It must, however, be confessed, that 
this is an extremely variable character in many sessile cirripedes. Internally the scutum 
(fig. 6¢) is chiefly characterised by the absence of characters, that is, by the slightness of 
the pits for the muscles, and the little prominence of the articular ridge. In the tergum 
(fig. 67), the spur is about half the width of the whole valve, and therefore rather wider 
than in 4. spongites. 
Genus—PYRGOMA. 
Pyrcoma. Leach. Journal de Physique, tom. 85, 1817. 
Boscta. Ferussac. Dict. Classique d’Hist. Naturelle, 1822. 
Savientum. Leach. Zoological Journal, vol. ii, July, 1825. 
Meraatrema. 0. Tb. 
ADNA. Ib. Ib. 
Daracta. J. #. Gray. Annals of Phil. (new series), August, 1825. 
Crevusta. De Blainville. Dict. Sc. Nat., Pl. 116, 1816-30. 
Nosra. G. B. Sowerby, junr. Conchological Manual,' 1839. 
Valve teste in unam confluente: basis cyathiformis aut subcylindrica, coraliis affizxa : 
valve operculares inter se articulate. 
Shell formed of a single piece: basis cup-formed, or subcylindrical, attached to corals : 
scutum and tergum articulated together. 
This genus can at once be recognised by the shell consisting of a single piece without 
sutures, whether viewed externally or internally, and by the cup-shaped basis, attached and 
often imbedded in corals. ‘The one species, P. Anglicum, found both recent and fossil, 
together with a closely allied recent species, P. Stokesiz, in all the characters derived from 
the opercular valves, closely resemble Balanus and other ordinary forms, and for this very 
reason they have some slight claims to be generically 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 Balanine and 
and Chthamaline taken together. 
1 The name, Nobia, is given in this work on the authority of Leach, but this must be a mistake, pro- 
bably caused by some MS. name, (that fertile source of error in nomenclature), haviug been used. 
