GENUS LEPAS. 69 



Terga, — flat, small compared with the scuta, usually of 

 an irregular quadrilateral figure, with the two upper or 

 occludeut margins very short, in proportion to the two 

 (carinal and scutal) lower margins ; all the margins are 

 nearly straight. The two occludent margins, generally 

 meet each other at about right angles, forming a small 

 triangular projection ; in L. fascicularis, however, the 

 occludent margin is formed by a single, slightly curved line. 

 The umbones (and primordial valves when distinguishable) 

 are not seated at the uppermost point, but at the angle 

 where the carinal margin unites to the upper of the two 

 occludent margins : during growth the terga are added 

 to, both on the occludent and on the scutal margins, and 

 slightly along the carinal margin ; hence their growth is 

 unequally qaaqua-versal, except at one angle of the irre- 

 gular quadrilateral figure. 



Carina. — This is always very narrow and curved, con- 

 cave within, often carinated and barbed exteriorly ; it ex- 

 tends upwards between the terga for one half or two 

 thirds of their length : at the lower extremity it ends 

 (with the exception of L. fascicularis), in a small fork 

 (PI. I, fig. 1, a, b) rectangularly inflected and embedded 

 in the membrane, beneath the basal margin of the scuta. 

 From comparing this lower part of the carina in L. aus- 

 tralis (fig. ha), with the same part in some of the species 

 of the allied genus Psecilasma, it would appear that 

 the fork is formed by an oblong disc, more and more 

 notched at the end, and with the rim between the two 

 points more or less folded backwards : conformably with 

 this view, in very young specimens of L. australis, instead 

 of a large and sharp fork, there is a small disc. The only 

 use of the fork appears to be to give firm attachment 

 to the membrane uniting the valves and peduncle. In 

 Z. fascicularis, instead of a fork, there is a broad, oblong 

 disc (figs. 6, 6a), rectangularly inflected; it is much longer 

 than the fork, in proportion to the upper part of the 

 carina ; the disc is not more deeply embedded than the 

 upper part. The umbo (and primordial valve when 



