OLENUS MUNDUS. 59 



Eyes of moderate size, set a little in front of the middle of the cheeks and distant 

 from the glabella about half the width of the latter, united with the anterior end 

 of the glabella by a strong ocular ridge which reaches the glabella just in front of 

 the first glabellar furrow. Facial suture running from the anterior margin back- 

 wards and slightly outwards to the eye, and thence more decidedly outwards to the 

 posterior margin, which it cuts nearly opposite to the end of the first thoracic 

 pleura. Margin narrow, even in width ; genal spines slender, short and sometimes 

 bent slightly outwards. 



Thorax of fourteen segments, considerably narrower than the head, the 

 decrease in width regular and gradual. Axis rather wider than the pleurae, bear- 

 ing a row of median tubercles (which are rather indistinct). Pleurae straight, the 

 fulcrum of the first segment placed about half way out, in the later segments 

 distant from the axis about two thirds the length of the pleurae ; pleurae bent 

 slightly downwards beyond the fulcrum, especially in the anterior segments, which 

 are facetted. The points of the first segments very short and directed outwards, 

 those of the later segments progressively longer and more and more backward in 

 direction, but all are short. 



Tail small, short, rounded or subtriangular. Axis conical, blunt, forming 

 about one third the width, reaching to the posterior margin, consisting of three or 

 four segments. Lateral lobes very slightly arched, marked by two fairly strong 

 furrows with a fine intermediate line. Margin sometimes slightly expanded at 

 the anterior angles, as if it bore a small spine. 



Dimensions. — Length 6 or 7 mm. 



The only species with which this form is likely to be confounded is Olenus 

 cataractes, and from this it is distinguished by its small size, the prominence of 

 the ocular ridges, the shortness of the pleural spines and the presence of tubercles 

 on the axis of the thorax. All these, except the last, are characters which might 

 reasonably be expected to disappear with age, and it is quite possible, therefore, 

 that 0. mundus is the young of 0. cataractes. It is the absence of intermediate 

 forms and the presence of the axial tubercles in the smaller form which prevent 

 me from accepting this view without further evidence. 



Olenus mundus was found by Mr. V. M. Turnbull at Trefgarn Bridge, 

 Haverfordwest, along with Olenus cataractes and several minute and immature 

 forms, which may be the young of either species. The material in which they lie 

 is scarcely fine enough to preserve the details of these larval forms, and the 

 margins of the heads and tails and the extremities of the pleurae are usually 

 lost or buried in the matrix. The most nearly perfect of the specimens is shown 

 in Plate VI, fig. 4, and in this the glabella extends forwards to the anterior margin 

 and is completely divided by transverse furrows into five segments, of which the 

 last appears to represent the occipital ring. The ocular ridge springs from the 



