50 PECTINID^E. 



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 Littorina, in the May 

 ' Annals of Natural History/ 1850, wherein 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 aiford 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. SIMIUS, Laskey. 

 P. similis, Brit. Moll. ii. p. 293, pi. 52. f. 6, and (animal) pi. 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 

 striae 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 



