TEREDO. 69 



typical specimens, (more so in foreign than in strictly 

 British exarai^les,) with an olivaceous epidermis, beneath 

 which it is equally devoid of colour with the rest of its 

 genus. The front triangular area is concentrically traversed 

 by elevated and rather closely-set striae, which diverge from 

 the anterior dorsal edge : to these succeed another series of 

 the most crowded and exquisitely engraved lines imagin- 

 able, which under a powerful glass exhibit a distinct mi- 

 croscopic subgranular decussation ; these latter, uninter- 

 rupted by either linear callosity or impressed striw, unite 

 almost at right angles with the former, and occupy a more 

 or less narrow triangular strip of surface, extending from 

 the beaks to the ventral tubercle. Posteriorly these fine 

 lines diverge, and form concentric and rather distant arches, 

 with occasionally intermediate striulro, which very quickly 

 become obsolete, leaving the hinder side comparatively 

 smooth and destitute of any decided sculpture ; there is, 

 nevertheless, a not unfrequent tendency in the surface of the 

 auricle and immediately adjacent parts to rise up in con- 

 fluent verrucosa granules. 



Internally, there is a kind of prolongation of the beaks, 

 in the shape of a protuberant callosity, which leans to- 

 wards the interior, and does not project above the dorsal 

 line ; this is terminated in the right valve by a narrow 

 shelf-like rim, and in the left by a projecting and recurved 

 tubercular lamina, which juts out rather obliquely from the 

 posterior side of the callosity. The subumbonal blade is 

 moderately but not peculiarly oblique, and presents its 

 broader side to the inner disc. It is very thin, and rather 

 wide, swelling out a little at its anterior edge, which is 

 simple and not jagged, but usually bending back again near 

 its termination, so that the apex is not broadly clavate, but 

 a little attenuated. The tubercle which terminates the 



