280 OSTREADiE. 



adult), armed more or less closely with fragile laminar 

 spines and vaulted or prickly scales, radiate from the urn- 

 bones to the ventral margin, intermediate smaller ones 

 starting up in the interstices directly the divergence of the 

 larger ones permits their developement. An alternation of 

 larger and smaller costellse is thus established. The inter- 

 stices, previous to the commencement of the intermediate 

 little ribs, exhibit, when perfect, somewhat concentric la- 

 minar wrinkles, but no microscopic chasing discovers itself 

 under a lens of high power. The ears are well denned 

 below, and densely covered with echinated costellse ; the 

 auricular sinus is subbiangulated, rarely profound or large 

 in the adult, but almost always more or less indicated ; 

 typically the cardinal angle of the front auricle is acute, 

 of the hinder one obtuse. An example that measures two 

 inches or an inch and three-quarters in width may be 

 regarded as a large individual, the majority of specimens 

 not averaging an inch and a half. It is difficult to deter- 

 mine which valve is the less shallow (neither is very convex, 

 and both are moderately strong) ; but the upper is gene- 

 rally regarded as rather the flatter. The interior, which 

 is not at all nacreous, is either white or partakes of the 

 external colouring. 



The animal is very similar to that of P. varius ; the 

 long cirrhi of the fixed margins of the mantle appear to be 

 longer and the eyes rather larger than in that species. The 

 mantle is most usually of a yellow hue, mottled or blotched 

 with brown ; the branchiae of a fawn-colour, and the body 

 of a pale pink tint. 



It is genei'ally distributed around our coast, ranging from 

 low-water mark, where it occurs at Herm attached to the 

 under-surface of rocks, whilst the young are free in the 

 neighbouring pools — (S. H.) — to a depth of fifty, sixty, and 



