386 MRS. E. W. SEXTON" ON A [Apr. 28, 



do not vary by the fraction of a millimetre. The intermedite 

 .specimen appeal's to be an immature female ; no incubatory 

 lamellae are developed, but the proportions of the antennal joints 

 agree with the adult form. 



Head. — Much deeper than the peraeon, as long as deep ; ros- 

 trum horizontal, apically rounded. In the adult the head is 

 slightly longer than the first segment of the perseon (3 mm. to 

 2"5 mm.) ; in the immature it is much longer ; and in the young 

 it is nearly twice as long (as 9 : 5). The huge Eyes cover nearly 

 the whole surface of the head ; they are composed of a great 

 number of small, dark brown ommatidia, arranged in rows, with 

 one row of unpigmented ommatidia following the contour of the 

 pigmented mass and meeting in the medio-dorsal line. There 

 are not less than 154 of the dark ommatidia, each side, but they 

 are too closely crowded together to permit of ascertaining the 

 exact number. 



In the immature the eyes are reniform, almost but not quite 

 touching dorsally, and not extending so far forward or downward 

 as in the adult ; not less than 120 pigmented ommatidia. In 

 the young a large number of unpigmented ommatidia. 



PERiEON. — The 1st segment of the perseon in the adult is the 

 largest, the four following subequal, the 6th and 7th the smallest, 

 all, except the first, produced at the posterior angle and rounded. 



Side-plates. — Ad.nlt (see perseopoda figures). The 1st is very 

 small, triangular, almost completely hidden by the 2nd, which 

 is greatly dilated inferiorly, forming a large triangular lobe, 

 reaching anteriorly to the infero-lateral margin of the head, 

 and produced to a great length downward posteriorly, slightly 

 emarginate behind. The 3rd is in shape obliqitely oval, anterior 

 mai'gin convex, posterioi' emarginate, less than half as wide 

 and slightly shorter than the 2nd. The 4th is shorter again, 

 rounded anteriorly and deeply emargii^ate posteriorly, inferior 

 margin truncate. The 5th is bilobed, posterior lobe deeper 

 and wider than the anterior ; 6th of similar construction, much 

 smaller. The 7th is the smallest, wider than deep, with rounded 

 corners. In the immature specimen the proportions of the 

 perteon segments and side-plates are the same as in the adult. 



In young. The first peraeonal segment is the largest, all the 

 others subequal ; body deep : side-plates not so large in propor- 

 tion as in adult, those of the second segment differing a little in 

 shape. 



Fh-EOi^.— Adult (PI. XYIII. fig. 1). The first three segments 

 large and subequal, the 1st pleon segment equalling the first 

 perfeonal segment in length (measured along the medio-dorsal 

 line). Epimeral plates large and rounded ; those of the 2nd 

 segment with a diagonal carina terminating in a denticle at the 

 postero-lateral angle, and those of the 3rd segment almost 

 rectangular ; 4th segment with a deep depression dorsally : 

 5th and 6th smaller, the 5th being slightly shorter than the 6th, 

 which is emarginate dorsally, for the insertion of the telson. 



