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 A. undulata, and likewise in A. spongites, I 

 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. 6d) 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. 6/), the spur is about half the width of the whole valve, and therefore rather wider 

 than in A, spongites. 



Genus — Pyrgoma. 



Pvrgoma. Leach. Journal de Physique, torn. 85, 1817. 

 Boscia. Ferussac. Diet. Classique d'Hist. Naturelle, 1822. 

 Savignium. Leach. Zoological Journal, vol. ii, July, 1825. 

 Megatrema. lb. lb. 



Adna. lb. lb. 



Daracta. J. E. Gray. Annals of Phil, (new series), August, 1825. 

 Creusia. De Blainville. Diet. Sc. Nat., PL 116, 1816-30. 

 Nobia. G. B. Sowerby , junr . Conchological Manual, 1 1839. 



Valves testa in unam confluente .■ basis cgathiformis aut subcylindrica, comliis affixa .• 

 valvce operculares inter se articulates. 



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. Stokesii, 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 Balaninae and 

 and Chthamalinse 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), having been used. 



