150 



OPHIOPSILA RIISEI. 



Ophiopsila Riisei. (Upper side.) 



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



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



Special Marks. — Arm-spines, six or seven. In the adult, many of 

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

 black sj)ecks. 



Description of a SjMcimen. — Diameter of disk, 8.6"™- ; 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-papilla, 



to that between outer comers of mouth- 

 slits, 1.6 : 1.6. Mouth-papillce, 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-papillae in an 

 irregular bunch, about nine in number ; 

 lowest ones smallest, and placed more out- 

 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- 

 der arm-plates broader than long, slightly 

 separated, nearly rectangular, but a little 

 wider without than within ; length to 

 breadth (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 bej^ond ; 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 loO 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 



Ophiopsila Riisei. (Lower siile.) 

 9, teiitacle-scal.; ; o, genital opening. 



