TECTEN. 297 



vex valve ; these are but moderately elevated, and are 

 rather wider than their interstices, which are simply 

 concave, and, equally with the costrc, sculptured with 

 large and distinct elevated radiating striae. These lat- 

 ter, however, are entirely absent upon the umbones of 

 the adult, and are not developed in the younger shells. 

 Similar stria? appear upon the flattened valve, but, to- 

 gether with the costa?, disappear upon the depressed umbo. 

 As the flat interstitial spaces of the upper valve corre- 

 spond to the broad ribs of the lower one, the converse 

 of the relative proportions of ribs and interstices here 

 prevails, the former (whose sides are abrupt, and whose 

 tops are but slightly rounded) being much narrower 

 than tjie latter, where the delicate crowded lamellar 

 striulse, which concentrically traverse the shell, are chiefly 

 manifest. 



The auricles are rather large, very nearly equal, sub- 

 rectangular, convex in the ventricose valve, concave in 

 the flat one : in addition to their concentric lamellar 

 striulse they are merely radiated with fine unarmed cos- 

 tellse or coarse elevated stria?. The auricular sinus is 

 nearly obsolete. The hinge-margin is almost level, but 

 rises slightly on cither side of the beaks in the upper 

 valve ; it is equal in the young to two- thirds the breadth 

 of the shell, and even in the adult is more than equal to 

 half the length, which latter is considerably superior to 

 the breadth of the valves. 



The prevailing tint of the flatter valve is rufous, with 

 some small angular markings of a darker hue upon the 

 ribs ; the umbo is of a yellowish white, or pale orange. 

 The convex valve is almost white ; sometimes too, to- 

 wards the umbo, of a roseate flesh-colour, marbled with 

 flexuous linear veins of white, and cinctnred with two 



VOL. II. a Q 



