﻿OPIIIOSTIGMA. 

 LIST OF SPECIMENS. 



103 



OPHIOSTIGMA LuTKEN. 



Type of the Genus, 0. isacanthum Lyman. 



Disk granulated. Teeth. No tooth-papilla?. Basal mouth-papillae 

 very long, stout, and broad ; the others small and few, arranged so as 

 to cover the end as well as the sides of the angle of the mouth. Arm- 

 spines three, short, smooth, arranged along the sides of the side arm- 

 plates. Side mouth-shields large ; nearly, or quite, touching, so as to 

 form a ring round the mouth. Mouth-apparatus, as a whole, forming 

 a distinct, raised pentagon. Two genital sUts, beginning outside the 

 mouth-shields. 



Ophiostigma isacanthum Lyjian. (Figs. 8, 9.) 



Opliiura imcanlha Say. Journ. Phil. Acad., V. p. 150. 1825. 

 Ophiostigma monillfonne Lutken. Addit. ad Hist. Oph., p. 132. 



Special Marks. — Upper arm-plates cleanly arched Avithout ; lower 

 arm-plates bounded within by an inner side and two inner laterals. 



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

 of mouth-shield to outer corner of opposite mouth-slit, 2.1°"°' ; width 

 of arm without spines, .8°"°- ; length of arm, 23.5'°°'- ; distance from 

 outer edge of mouth-shield to inner points of teeth, to that between 

 outer corners of mouth-slits, 1 : 1.2. Mouth-papilte, four to six. Teeth 

 four, stout, short, squarish, with the cutting edge a little re-enteringly 

 curved. Mouth-shields broad heart-shaped, with a sharp peak mthin, 



