180 " THETIS " SCIENTIFIC RESULTS. 



The specimen exhibits a few characters worth recording. The 

 frontal process is bordered by a distinct and sHghtly thickened 

 margin, the lateral ridges are, like the median ridge, rounded 

 along their summits and are a little longer, but not quite so wide; 

 the lateral depressions bounding the median ridge bifurcate at 

 their posterior half; the branches from thence follow the contour 

 of the ridges to their terminations. 



Each dactylus of the ambulatory legs has at the lower proximal 

 base a broad, transverse, convex prominence, the anterior margin 

 of which is rounded, the posterior neatly emarginate ; the outer 

 angle is prolonged and extends upwards as a gradually narrowing 

 ridge to the margin of the upper boi'der. The whole surface of 

 this peculiar prominence is minutely scaly, roughened and quite 

 unlike the adjoining surface, which is smooth. 



The lower distal extremity of the propodal joint has a slightly 

 elevated rim, which is semicircular in outline, swollen, and 

 bounded behind by a shallow groove ; it has an elongate lobe on 

 the outer side, which is, when the tarsi are drawn back — closely 

 applied to the corresponding lobe on the outer side of the tarsus; 

 the anterior concave surface of the rim and its lobe are roughened, 

 scaly, and adapted to the scaly prominence on the base of the 

 tarsus. The two roughened surfaces when superposed would 

 have a brake-like effect, and form a kind of locking joint. 



Judging by the appearance of the body and limbs, it seems 

 highly probable that this species lives in the shells of Dentalium 

 or shells of narrow dimensions ; the second and third legs are 

 parallel with and closely applied to the sides of the cheli pedes. 

 If such be the habitat, the function of the specially modified 

 tarso-propodal joint may be easily explained. I venture to 

 suggest that the function is opercular and that the tarsi are used 

 to protect the entrance of the shell from would-be intruders. Tt 

 appears probable that when the tarsi are drawn back and at right 

 angles to the propodi — beyond which angle they cannot be forced 

 without breaking — that the two roughened surfaces are brought 

 into close contact and the joints locked ; the claws would then 

 form a kind of rigid four-railed fence across the aperture of the 

 shell. The chelipedes are of such a length that the long, narrow 

 fingers could be projected between the tarsi and give a warning 

 nip to any organism attacking from without. The tarsal joint is 

 slightly curved and about 2-5 mm. long ; the lower border is 

 armed with two or three strong calcified spines O'l to 0*2 mm. 

 long and a few stiff setse, and terminates in a stoutish black horny 

 claw. 



Shorter ramus of pleopoda with a rudimentary rasp of four 

 curved scales. 



The rami of the uropods have the margins more or less sub- 

 serrate, each serration tipped with a long plumose hair; the outer 



