454 PEOF. p. M. DUNCAN" ON SOME 



with curved sides within, and with straight sides and a straight 

 or curved edge aborally. In small specimens there is a small scale 

 quite in the angle of the radial shields. A ridge, more or less 

 dentated witli minute spines, is on each side, and it is continuous 

 with one on the next arm-plate, and it merges into the upper 

 spines of the side arm -plate. The second arm -plate is broader 

 than the other, broadest and curved without, with sides sloping 

 to the smaller oral end. The next plate, also nearly quadran- 

 gular, is broader than long, slightly curved without, and nearly 

 straight within. The fourth plate is hexagonal, broader than 

 long, broadest without ; and the edges are straight at the sides, 

 and curved elsewhere slightly. The other upper arm-plates are 

 hexagonal, longer than broad, with the greatest width externally ; 

 and towards the tip the length increases and the edges within and 

 Avithout become almost points. All are slightly convex from side 

 to side, giving a keeled shaj^e to the top of the arm. 



The side arm-plates are important members of the arm ; but 

 they form the tall sides, and but little of the upper and lower 

 surfaces. They are therefore tall, broad, and slightly curved at 

 the free edges. They lie close to the side of the arm, where 

 their breadth is very equal ; and on their oral side, beneath the 

 overlapping, ill-developed, semllamellar spines of the outer edge 

 of the antecedent plate, there is a row of linear striations corre- 

 sponding apparently with the spines. Above, there is an angular 

 process of the side arm-plate which articulates with the sides of 

 the upper arm-plates ; and quite at the end the side arm-plates 

 meet above. 



On the lower part of the arm a very slender process of the side 

 arm-plate is in contact externally with the second lower arm- 

 plate ; and the next and following side arm-plates join the lower 

 arm-plate on the increasingly wide distal lateral side of the hexa- 

 gon. In mid arm the side arm-plates encroach more on the arm- 

 plates, and finally near the end separate them and unite. They 

 form much of the under part of the arm ; and they are swollen 

 and convex towards the under part of it. The tentacle-open- 

 ings down the arm are rather large, linear and broad, and in- 

 crease in size tow^ards the disk. Low down the arm there are only 

 rudimentary tentacle-scales on the side arm-plates. The spines 

 are on the edge of the side arm-plates, and do not project out- 

 wards; they are thin, lamellar, or rarely pointed, and are numerous 

 occasionally when the ordinary lamellar condition is split up. 



