124 AMPIIIURA TENERA. 



inward ; length to breadth, .3 : .3. Side mouth-shields proportionately 

 very large, meeting within, shaped like regular wedges, with the broad 

 end out. Under arm-plates a trifle longer than broad, shield-shaped, 

 with an angle within ; outer side nearly straight, lateral sides a little 

 re-enteringly curved, outer corners rounded, length to breadth (sixth 

 plate) about .3 : .3. Side arm-plates encroaching above and below, so 

 as to separate both upper and under arm-plates. Upper arm-plates Avith 

 outer side slightly curved ; inner and lateral sides included in a common 

 curve, which has the form of an arch ; length to breadth (third plate) 

 about .3 : .4. At the tip of the arm, the joints are much longer than 

 broad, wide without, but greatly constricted within ; they are embraced 

 by the side arm-plates, the upper and under plates l^eing small in size, 

 and occupying the outer end of the joint. Disk-scales pretty regular 

 and even ; above, about 36 to a square mm. ; below, considerably 

 smaller ; along the margin of the disk, the free edges of the lower 

 scales make a distinct line. Radial shields shaped like elongated pear- 

 seeds, with one side nearly straight ; joined for their whole length, 

 except their inner points, which are separated by a single scale ; length 

 to breadth, .7 : .2. Arm-spines three (close to disk sometimes an addi- 

 tional one above) ; uppermost and lowest nearly equal, short, rounded, 

 a little flattened, tapering, rather sharp ; lengths to that of under arm- 

 plate, .3 : .3 ; middle one a trifle shorter, rounded, not flattened, swelled 

 at the base, and contracting suddenly to a point. Close to tip of arm, 

 there are two or three spines, of which the lowest is longest ; they are 

 sharper and more elongated than those near base of arm. Tentacle- 

 scales two, of moderate size, standing partly erect, and at an angle to 

 each other. Color, in alcohol : nearly white, with faint traces of 

 brownish on disk. 



Variations. — The disk may be to the arms as 3 : 12, 2.7 : 10, or 

 2 : 6.6. Otherwise the specimens vary scarcely at all. A young one, 

 with a disk of 2"""-, had the under arm-plates proportionately longer, 

 and the side arm-plates more developed ; the disk-scales were in like 

 manner larger, and the primary plates more conspicuous. 



A. tenera is distinguished from A. squamata and A. pugetana by its 

 swollen middle arm-spines ; from A. Orstedii, A. violacea, and A. mi- 

 crodiscus, by different upper arm-plates ; and from A. Puntarence by 

 wanting a notch in the lower arm-plates. It has been taken at St. 

 Thomas in four fathoms (A. H. Riise). 



