_Terga_,--flat, small compared with the scuta, usually of an irregular quadrilateral figure, with the two upper or occludent 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 _quaqua-versal_, except at one angle of the irregular quadrilateral figure. _Carina._--This is always very narrow and curved, concave within, often carinated and barbed exteriorly; it extends 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 (Pl. 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. australis_ (fig. 5 _a_), with the same part in some of the species of the allied genus Pæcilasma, 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 _L. fascicularis_, instead of a fork, there is a broad, oblong disc (figs. 6, 6 _a_), 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 distinguishable,) of the carina is seated just above the embedded fork (or disc in _L. fascicularis_), at the point where the inflection takes place; hence the main growth of the carina is upwards,--the fork, however, being of course, likewise added to at its point: in _L. fascicularis_, the growth is both upwards and downwards.