50 PECTINID 2. 
lengths ; the mantellar marginal areas are a blotchy mixture 
of fawn, yellow, pale red-brown, and lead colours. 
This species comes much closer to the type than P. varius. 
From the great variableness of the colours in the same species, 
and the general similitude in the structure of the organs, it is 
sometimes difficult to seize the specific distinctions of closely 
allied animals; in such case we must call in aid the figure and 
markings of the shell, which being dependent on the dispo- 
sition of the secreting glands of the mantle, cannot well be 
malacologically appreciated. This case strongly supports the 
views laid down in the paper on the Littorine, m the May 
‘Annals of Natural History,’ 1850, wherem we strenuously 
contend for the examination of both the animal and the shell 
to ensure a true distinction of species, as when the soft parts 
do not afford sufficient characters, the shell may assist in re- 
solving the difficulty. 
This species is usually imbedded in masses of Flustra and 
old bivalves by the under valve. 
P. sturuis, Laskey. 
P. similis, Brit. Moll. 1. p. 293, pl. 52. f. 6, and (animal) pl. S. f. 1. 
Shell suborbicular, depressed, but equally convex on both 
surfaces ; the auricles are not greatly dissimilar, the posterior, 
however, is the smallest, their terminations are sometimes 
rounded and sometimes obliquely truncate. There is very 
little sculpture on either valve beyond the fine subcircular 
strie of increase, and rarely there are faint traces of ribs, but 
the markings on both surfaces are very variable in figure and 
colour; they exhibit every combination of blotches of all hues, 
and sometimes a series of markings simulating radiating ribs 
so closely as to be difficult of detection ; but these appearances, 
under proper optical powers and light, will be found to be in 
the test, with the surface quite smooth. 
We have taken such specimens alive in the coral zone at 
Exmouth, from one of which are derived the notes on the 
animal. Diameter less than a quarter of an inch. 
Animal of the same form as the shell, and as variable in its 
