OLENUS CATARACTES. 57 



width of the latter, united with the anterior end of the glabella by a very faint 

 ocular ridge. Facial suture running from the anterior margin backwards to the 

 eye, and thence backwards and outwards to the posterior margin, which it cuts 

 some distance within the genal angle. Margin narrow, even in width ; genal 

 angles produced into spines which are directed backwards and are about as long as 

 the head. 



Thorax of fourteen or fifteen segments, decreasing in width from the sixth or 

 seventh segment backwards. Axis about equal in width to the pleurae. Pleurae 

 nearly straight and horizontal, obliquely grooved ; the fulcrum of the first segment 

 placed about half way between the axis and the extremity of the pleura, in the 

 following segments successively further out until in the seventh or eighth segment 

 it has disappeared; outside the fulcrum the pleura? are slightly bent downwards 

 and facetted, and end in short points ; beyond the seventh segment there is no 

 facet and the extremities are produced into spines directed backwards, the spine 

 of the tenth segment being the longest. 



Tail small, sub-triangular. Axis forming about one third of the whole width, 

 conical, terminating bluntly a little in front of the margin, composed of four 

 segments (including the terminal portion). Lateral lobes flat, with three broad, 

 shallow, outwardly-directed grooves, of which only the first is well defined ; the 

 ridge between the first two grooves marked by a narrow impressed line. Margin 

 narrow, even, bearing on each side near the anterior angles a short, sharp, 

 backwardly-directed spine. 



Dimensions. — Very variable, length from 10 — 40 mm. 



From Olenus truncatus, 0. gibbosus, and 0. longispinus this form is readily 

 distinguished by the length of the glabella, which reaches forwards nearly to the 

 anterior margin, and this character is easily recognised even in very imperfect 

 specimens. Olenus micrurus presents a closer resemblance, and is distinguished 

 chiefly by the rapid narrowing of the hinder part of the thorax, the smallness of 

 the tail, and the presence of only one distinctly differentiated ring upon the 

 pygidial axis. 



It often happens that in distorted specimens Olenus cataractes presents a 

 striking general resemblance to Parabolina spinulosa, and this is especially the 

 case when the specimen is laterally compressed. The thoracic segments then 

 appear to terminate in long spines, and even the tail may seem to bear several 

 spines, while the characters of the head are not strikingly different. It will be 

 observed, however, that the eye is placed considerably further back than in 

 Parabolina spinulosa, and the thorax is not provided with a row of tubercles upon 

 the axis. These characters will usually serve to distinguish between the two 

 forms in all but the most fragmentary specimens. 



But the form with which 0. cataractes is most nearly allied is 0. mundus, 



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