﻿150 OPIIIOPSILA RIISEI. 



Ophiopsila Riisei Lutkex. (Figs. 16, 17.) 



Opliiopsila Riisei Lutkex. Addit. ad Hist. Oph., p. 13G. 1859. 



Sj^ecial Harks. — Ai-m-s^sines, six or seven. In the adult, many of 

 the upper arm-plates split in two. Color, in alcohol : gray, with fine 

 black specks. 



Description of a Specimen. — Diameter of disk, S.G""- ; from outer 

 side of mouth-shield to outer corner of opposite mouth-slit, 3°"°" ; width 

 of arm without spines, 2°"°-. For length of arm, see Variations. Dis- 

 tance from outer side of mouth-shields to inner points of tooth-papillaa, 

 to that between outer corners of mouth- 

 f'° ^"^ slits, 1.6 : 1.6. Mouth-papillge, six to each 



corner of the mouth, of which the outer 

 one is a minute tubercle, scarcely percep- 

 tible ; the two inner ones on either side 

 large, flat, and square. Tooth-papillse in an 

 _^^Ji^^" irregular bunch, about nine in number ; 



„ , , „ „ , , lowest ones smallest, and placed more out- 



OijhiopoiH Rusei (I pper side ) '1 



ward. Teeth five, four upper ones broad, 



short, thick, even, squarish ; lowest one narrower, and more like a large 



tooth-papilla. Mouth-shields broader than long, oval diamond-shaped, 



with a peak within ; the outer side curved, and the lateral ends more 



or less truncated ; length to breadth, .7 : 1. Side mouth-shields scarcely 



to be seen, even in dried specimens. They seem to run along the inner 



side of the mouth-shield, but are extremely narrow, and are often quite 



confounded with the surrounding parts. Un- 



_^!j^!_^ der arm-plates broader than long, slightl^r- 



,^^\ "' , separated, nearly rectangular, but a little 



wider without than within ; length to 

 l)readth (fifteenth plate), .5 : .7. Upper 

 arm-plates broader than long, nearly oval, 

 somewhat angular, however, at the corners ; 

 many of them broken lengthwise in two 

 pieces ; the first six or eight plates are 

 proportionately smaller, narrower, and more 

 rounded than those just beyond ; length to 

 breadth (third plate), .5 : .7 ; (twenty-third 

 plate), .6:1. Disk-scales covered with a thick skin, so that they are 

 not easy to see, even in a dried specimen ; about 150 to a square mm. 

 where they are finest ; over the places of the radial shields, somewhat 

 coarser. Arm-spines usually six, near the disk seven ; cut square off at 



