282 OSTREAD^. 



bulging out at tlie hinder extremity. The valves are 

 shallow, but nearly equally convex ; their exterior is shin- 

 ing, and microscopically chased with dense and peculiarly 

 irregular divergent wavy radiating striulse, which, when 

 examined under a very powerful glass, often appear subar- 

 ticulated. The upper valve (whose colouring is likewise 

 the darker) exhibits also very numerous but not much 

 elevated striae, which run from the beaks to the ventral 

 margin, and are echinated (at least in the younger speci- 

 mens) by short broad and minute prickly scales, which 

 are most apparent towards the lower margin, but, where 

 abraded, show traces of their former presence by the some- 

 what articulated or catenulated appearance of the striae. 

 This sculpture, which extends likewise to the auricles, is 

 obsolete or very obscure upon the lower valve, where 

 microscopic concentric lines more or less decussate and 

 shagreen the minute striulse. The colouring and style 

 of painting is not unlike that of tigrinus, consisting of 

 whitish streaks, clouds, or spots, on a ground of brownish 

 or somewhat livid red, varying in intensity from chocolate 

 to pale brownish flesh-colour ; snow-white flakes or minute 

 specks occasionally augment the beauty of the pattern. 

 There exists also (as in most Pectens) a colourless variety. 

 The dorsal slopes are incurved and rather short ; the 

 hinge-margin, which is almost invariably equal to half the 

 length of the shell, often exceeds that proportion ; the 

 ventral margin is rather broadly rounded (in the adult) ; 

 the apices of the valves acute and prominent. The 

 auricles, which are well defined in both valves, are very 

 unequal. The posterior one, although not very small, is 

 only half as long as the other ; its base, however, reaches 

 as low down as that of the hinder one ; its angle is 

 decidedly obtuse ; its side straightish, and occasionally 



