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



are separated from the rest by a ridge which runs from just below the rostrum to the 

 lateral margin on either side ; the whole cephalic shield is very distinctly separated from 

 the thoracic segment by a deep furrow ; the central portion which lies between the eyes 

 is very strongly convex, and. separable into three regions — two round convexities which lie 

 to the inner side of and behind each eye, and a median T-sliaped elevation, at the upper 

 end of which, on a level with the anterior portion of the eyes, are four tubercles arranged 

 in a semicircle with the concavity directed forwards ; at the hinder extremity is another 

 short tubercle ; the whole surface of the cephalic shield, with the exception of a flattened 

 area which extends from the upper end of the eyes to the transverse ridge, is covered with 

 minute pit-like depressions. The eyes are whitish yellow in colour. 



Thorax. — The thoracic segments, like the cephalic shield, are covered with an immense 

 number of irregular pit-like depressions ; the posterior margin of each segment is furnished 

 with a minute tubercle. The epimera are of great length, and increase gradualh' from 

 the first up to the sixth and last pair. 



The epimera of the first thoracic segment are as usual large and expanded, but narrow 

 rapidly towards the distal extremity, wliich is narrow and spine-like as in the succeeding 

 segments. The anterior margin of these epimera .bears a short forwardly directed spine at 

 about 3 mm. distance from the cephalic shield. The epimeron is divided into three pieces 

 by a Y-shaped ridge ; the two arms of the Y fomi a very obtuse angle, one is continuous 

 with the anterior spine already referred to, and the other passes outwards towards the 

 distal end ; the unpaired arm seems to correspond to the suture which is often developed 

 upon the first pair of epimera in other species. The two areas lying respectively in front 

 of and behind this median ridge are somewhat concave, while that portion which lies to 

 the outside of the arms is convex. 



The epimera of the three succeeding segments are curved and sickle-shaped, and 

 project outwards at a greater angle with the longitudinal axis of the body than the rest, 

 which by degrees come to lie almost in the same straight line with this axis. A distinct 

 suture separates the dorsal portion of the three anterior free thoracic segments from these 

 epimera. All the epimera of the body project downwards as well as outwards, especially 

 in the male specimen, so that when the animal is placed upon a flat surface, the body 

 rests entirely upon the epimera. The second of the free thoracic segments is the widest, 

 and measures 4 "5 mm. in diameter ; the first and third are a trifle smaller, while the 

 fourth and fifth are only one-third of the diameter of the second. 



The sterna of the two first thoracic segments are divided by sutures into three portions, 

 a median and two lateral. The median portion of the anterior segment (that which bears 

 the maxilhpedes) is keeled ; the sterna of the second, third, and fourth segments are 

 divided by a median suture into two equal halves ; the remaining segments have a rather 

 peculiar arrangement, which is displayed in PL IV. fig. 2 ; the middle portions of the 

 three segments are fused together to form a somewhat oval plate, divided by a median 



