16 G-. O. Sårs. 



The carapace has now been transformed into a distinctly 

 bivalvular shell, obtecting the trunk not only dorsally but 

 also laterally, so as to include between the valves both the 

 oral parts and the legs. It is, however, still of inconsider- 

 able size, as compared with that in the adult animal, for 

 which reason, the anterior and posterior parts of the body 

 remain uncovered, projecting considerably beyond it, without 

 being enabled to withdraw themselves into the shell. 

 Seen laterally, it exhibits a rounded triangular form, its 

 antero-dorsal corner being somewhat projecting. The dorsal 

 edge, where the valves are connected, appears rather short 

 and nearly straight, joining the free posterior edges of the 

 valves at an obtuse angle. In the anterior part of each 

 valve the shell-gland may be very distinctly traced as an oval 

 coil of closely crowded canals. Otherwise the shell is quite 

 smooth, without any trace of concentric lines, and highly 

 pellucid. 



The anterior, or cephalic part of the body is still 

 strongly convex above, and is defined dorsally from the ad- 

 joining part of the trunk by a distinct, though not very 

 deep depression, from which a transversal suture passes on 

 each side to the insertion of the mandibles. The frontal 

 part of the head is now produced downwards to a distinct, 

 though not very prominent, acute rostrum, and has the an- 

 terior edge someAvhat irregularly flexuous. Ventrally, the 

 head is prolonged into the posteriorly pointing labrum, the 

 form of which now nearly agrees with that in the adult 

 animal, being somewhat compressed, not, as in the larval 

 stages, shield-like. 



The compound eye is still double, and has the pigment 

 rather poorly developed, the visual elements being not yet 

 distinctly formed. On the other hand, the ocellus, located 



