OPIIIURA BREVISPINA. 19 



shields long heart-shaped, the point inward ; length to breadth, 1.4 : 

 1.2 ; madreporic shield wider, with an uneven surface ; side mouth- 

 shield.s, connecting first under arm-plate with mouth-shield, irregular, 

 lengthened, triangular, swelled ; under arm-plates hexagonal, with cor- 

 ners somewhat rounded; outer side of each plate overlapping inner side 

 of its successor; first plate somewhat oval, with two notches in its outer 

 side, where two pores enter, between it and the second plate; length 

 to breadth, 6 : 12 ; second plate, outer side curved, lateral sides re-enter- 

 ingly curved to receive tentacle-scales, inner side a little re-enteringly 

 curved, with a small tongue running inwards under first plate ; the 

 third plate, and those just beyond, have a similar shape, but the outer 

 curve may be broken up into three straight lines, of which one is the 

 outer side proper, and the other two are the outer laterals ; the inner 

 side is bounded by two inner hiterals, which start from tentacle pores 

 and disappear under preceding plate ; farther out, lateral sides become 

 straighter, and outer side is a simple curve ; near tip of arm, plates 

 do not overlap, lateral sides are short and straight, while inner lat- 

 erals are very long and gradually approach each other, to meet on 

 the middle line of the arm ; lengths to breadths as follows : second 

 plate, 1:1; third plate, .8:1; tenth plate, .8:1; about tw^o thirds out 

 on arm, .6 : .4 ; close to tip of arm, .4 : .3. Side arm-plates do not meet 

 above or below, even at tip of arm ; the outer edge of each is on a 

 line with the middle of an under arm-plate ; above, they overlap the 

 iiuier corners of upper arm-plates a little ; upper arm-plates much 

 broader than long ; outer and inner sides nearly straight ; lateral 

 sides, being partly overlapped by side arm-plates, seem to slope from 

 without inwards ; third plate with outer and inner sides nearly 

 straight ; lateral sides sloping inwards ; corners rounded ; length, 

 .8"'" ; outer side, 1.8"""-; inner side, 1.4"^'"- ; this is the typical shape of 

 most of the upper arm-plates ; fifteenth plate, length to breadth, 

 .8 : 1.6 ; outer side curved ; fortieth plate, length to breadth, .6 : 1.2 ; 

 close to tip of arm, plates long heart-shaped, the point inward, length 

 to breadth, .4 : .4 ; disk, above and below, together with spaces between 

 mouth-shields and tooth-papilla3, evenly and closely covered with 

 grains, about 260 to a square mm. Genital slits bounded by granula- 

 tion of disk ; between outer ones and side of arm may be seen grains 

 and the edges of some scales. Arm-spines somewhat flattened, of 

 nearly equal length, points rounded, length to that of under arm- 

 plates, .6 : .8 ; sixth joint, five spines ; seventh joint, six spines ; eighth, 

 ninth, and tenth joints, eight spines ; eleventh joint, seven spines ; 

 about two thirds out on arm, six spines ; close to tip of arm, three 

 spines, shorter than joints, and more rounded than at base of arm. 

 Tentacle-scales flat, with rounded ends, the one next to under arm- 

 plate the larger ; there are two to each tentacle, almost to the tip 



