68 GENUS LEPAS. 



either moderately thick and translucent, or very thin and 

 transparent; and hence, though themselves colourless, they 

 are often coloured by the underlying corium. Their sur- 

 faces are either smooth and polished, or striated, or fur- 

 rowed, and sometimes pectinated. They are not subject 

 to disintegration ; they are generally naked, except on the 

 borders, where they are coated, and held together by 

 membrane ; in L. fascicularis, however, the valves are 

 covered with thin membrane, bearing very minute spines. 

 The manner of growth of the valves will be best des- 

 cribed under each. All the valves, even in the same 

 species, are subject to considerable variation in shape, 

 more especially the terga. 



Scuta. — These valves are sub-triangular in outline, 

 with the basal margin straight and rather short; and 

 with occludent and tergo-carinal margins more or less 

 protuberant ; in L. fascicularis, however, the basal (PL I, 

 fig. 6), and occludent margins are slightly reflexed and 

 prominent. A ridge, generally runs from the umbo to the 

 upper point. Internally, there is no conspicuous pit for the 

 adductor muscle ; under the umbones, there is generally 

 either on both valves, or only on the right-hand side 

 (PL I, fig. 1 <?), a small calcareous projection or tooth, of 

 variable size and shape, even in the same species ; it is 

 generally largest on the right-hand valve ; these teeth at 

 first sight appear to form a hinge, uniting the opposite 

 scuta at their umbones, but this is not really the case, 

 and their use appears to be only to give attachment to 

 the membrane uniting the valves together, and to the 

 peduncle. The basal margin is internally strengthened 

 by a calcified rim, more or less developed. The umbones 

 (and primordial valves when distinguishable,) are seated 

 at the rostral angles; during growth the basal margin 

 is not added to, and the occludent margin only to small 

 extent ; hence the main growth of the valve is at the 

 upper end, and along the carina-tergal margin. In 

 L. fascicularis, however, the basal reflexed margin is 

 slightly added to beneath the umbo. 



