500 THE VOYAGE OF H.M.S. CHALLENGER. 



smooth, and runs continuously to the posterior margin, where it unites with the infra- 

 lateral angle of the carapace, except where it is interrupted by a very small tooth opposite 

 the penultimate somite of the pereion. 



The pleon has the lateral margins of the five anterior somites produced to an angle. 



The first somite is dorsally armed on the anterior margin with two anteriorly directed 

 cusps, one on each side of the median line, which is smooth, outside which are two 

 others, less regular in form ; the posterior margin is tolerably smooth. The second somite 

 is armed with a strong, pointed, anteriorly directed, central tooth, which produces a 

 carina in the median line for half the length of the somite ; it is transversely divided 

 by a groove, posterior to which the somite is smooth, the lateral surface only being 

 broken up on each side into two anterior and posterior cusps. The third somite is carinated 

 throughout its entire length, the transverse groove or sulcus terminating on each side 

 close to the central carina ; the lateral surface is divided into small cusp-like elevations. 

 The fourth somite is carinated throughout its entire length, the transverse sulcus ter- 

 minating at the central carina, from near the middle of which a lateral carina runs 

 obliquely to the posterior margin. The fifth somite has no central carina, but one on 

 each side runs obliquely backwards and outwards from near the anterior to the postero- 

 lateral margin, and each is interrupted so as to form a sharp tooth near its centre. The 

 sixth somite has no central carina but a longitudinal groove or depression, chiefly due 

 to the lateral elevation which forms a longitudinal carina on each side, which is armed by 

 three sharp teeth in a line ; the postero-lateral angle is produced into a strong tooth on 

 each side. 



Each of the five anterior somites is produced laterally to an obtuse angle which is 

 bossed on the outer surface. The sixth somite differs from the others in having the 

 posterior angle pointed and directed backwards, and the lateral angle is also sharper than 

 in the preceding somites, and situated more posteriorly, while the boss or tubercle, 

 instead of immediately surmounting the angle, is situated near the frontal margin. 



The telson is long, narrow, acuminate and dorsally flattened, and the lateral margins 

 are abruptly depressed. 



The ophthalmopoda are short, large, round, and project but slightly beyond the 

 rostrum. The first pair of antennas is short ; the first joint, the outer distal angle of 

 which projects into a sharp tooth, reaches but little beyond the extremity of the ophthal- 

 mopoda. It is concave on the upper surface for the reception of the ophthalmopoda. 

 and carries on the outer side, attached to the base, a stiff and pointed stylocerite, the apex 

 of which reaches nearly as far as the distal extremity of the joint ; a fasciculus of long 

 cilia is implanted within the margin, and from its position and form is apparently 

 adapted for sweeping the surface of the ophthalmus and protecting it from 

 external injury. The second joint is short, scarcely as long as the anterior margin is 

 broad, and is distally produced externally to a sharp point or tooth. The third joint is 



