Structure of the Parietes and Radii.—The parietes are not so thick as in [E. plicatus]; internally they are tinted pale purple; when broken transversely, a row of microscopically minute orange-coloured dots can generally be distinguished between the outer and inner laminæ; and these evidently represent the orange-coloured layer in [E. plicatus]. The sheath also exhibits a faint tinge of orange. The radii are very narrow, and are quite smooth-edged, differently from in [E. plicatus]. The edges of the alæ barely exhibit a trace of being crenated.
In the body I could perceive no difference from [E. plicatus], excepting that in the third pair of cirri the two rami are like each other, and do not support any coarsely pectinated, only serrated, spines; but after what we have seen on the variability of these very same characters in [Tetraclita porosa], I dare not trust to them. The three posterior pairs of cirri, also, seem here to be more elongated in proportion to the others, than in [E. plicatus].
Affinities.—It is certain that this species is most closely allied to [E. plicatus]; but as I have seen many specimens of the latter brought by different persons from New Zealand, and as I have observed in them no approach to the characters of [E. simplex], which, in specimens from three localities, also appear to be constant, I have considered the two forms as specifically distinct. The present species differs from [E. plicatus], in its white, conical, moderately ribbed, well preserved, smaller shell; and more especially in the orange-coloured intermediate lamina of [E. plicatus] being here represented only by microscopically minute dots. But the radii being smooth-edged, is the most important differential character, though in [E. plicatus], during its earliest growth, whilst still immature and colourless, the radii are likewise smooth-edged.
5. Genus—PYRGOMA.
PYRGOMA. Leach. Journal de Physique, tom. 85, 1817.
BOSCIA. Ferussac. Dict. Classique d’Hist. Naturelle, 1822.
SAVIGNIUM. Leach. Zoological Journal, vol. 2, July, 1825.
MEGATREMA. Ib. Ib.
ADNA. Ib. Ib.