OLENUS. 



51 



will be found useful. It shows the distinguishing features of the more important 

 genera which are commonly included under the name of Olenus sensu lato. 



( 8. Acerocare. — Tail entire. 



1 7. Peltura. — Tail spined. 



' 0. Sphaerophthalmus. — Glabella con- 

 siderably higher than the strongly 



I. Inermes. — No cheek-spines 



II. Abruptae. — Cheek- 

 spines abruptly 

 projecting from 

 the outer bor- 

 der of the 

 cheek 



' (a) Pleurae with 

 Ion 



m- 



(b) Pleura with 

 short spines 



III. Continuse. — Cheek-spines in direct 

 continuation of the outer margin 

 of the cheek .... 



arched cheeks. 

 5. Gtenojyyge. — Fixed cheeks flat, 



clined to the glabella. 

 -1. Leptoplastus. — Cheek - spine short, 



slightly bent. 

 3. Eurycare. — Cheek-spine long, broad, 



strongly bent. 

 2. Parabolina . — Strong spines to tail. 

 1. Olenus. — Tail entire, or with rudi- 

 mentary spines. 



In the section Abruptae the cheek-spines originate well in front of the posterior 

 margin and spring abruptly from the outer border of the head, making a distinct 

 angle with, the exterior margin. In the Continuse the cheek-spines, as in most 

 Trilobites, arise at the genal angles, and the outer edge of the spine is the direct 

 continuation of the outer border of the head. 



In Persson's classification no place is found for Brogger's sub-genus Para- 

 bolinella. It is, however, one of the Continuse, and as the tail is without spines it 

 falls into the same division as Olenus proper. In several other respects it is more 

 closely connected with Olenus than with Parabolina. 



Limiting the genus Olenus to the species similar to Olenus gibbosus it may be 

 defined as follows : 



General form depressed, ovate. Head nearly semi-circular, with the genal 

 angles produced into spines, which are in direct continuation of the outer margin 

 of the cheeks. Glabella nearly rectangular, but narrowing slightly towards the 

 front, with two or three pairs of oblique glabellar furrows. Facial suture running 

 from the anterior margin backwards to the eye, and thence backwards and out- 

 wards to the posterior margin, cutting the latter some distance within the genal 

 angle. Eyes placed slightly in front of the middle of the cheek, and some 

 distance from the glabella, connected with the glabella by a distinct ocular ridge. 

 Thorax of 1-3 — 15 segments ; the anterior pleurae facetted, slightly bent downwards 

 beyond the fulcrum, the posterior pleurae nearly horizontal, without fulcrum or 

 facet ; all the pleurae spined and grooved. Tail small, semi-circular or triangular, 

 sometimes entire, sometimes with a small spine at the anterior angles. 



