BRITISH FOSSILS. 



Decade II. Plate II. 



ILL^NUS DAVISII. 



[Genus ILL^ENUS. Dalman. (Sub-kingdom Articulata. Class Crustacea. Order 

 Entomostraca. Family Trilobitse.) Head about a quarter of a sphere ; eyes smooth, 

 remote ; facial suture ending on the posterior margin behind the eye, marginal or sub- 

 marginal in front ; hypostome entire, separated from the facial suture by a transverse 

 rostral shield ; thorax segments 9 or 10.] 



[Subgenus Illcenus. Axis of thorax separated from the pleurae by distinct furrows; 

 fulcrum not close to the axis.] 



Diagnosis. /. capite pygidioque cequalihus ; oculis parvis prope stdcos 

 axales^ a margine occipitali diametrum et plus distantibus ; thorace seg- 

 mentis 10, axi pleuris (pquali ; fulcro in segmento primo ad axin approxi- 

 mator in postremo dimidium pleurcB vix attingente. 



Synonyms. /. Bowmanni [Specimens from Rhiwlas only], Salter 

 (1845), in Geol. Journ., vol. i., p. 20 ; also p. 8. Illcenus crassicauda, 

 Sharpe (1848), Geol. Journ., vol. iv., p. 149. 



Description. — Length 2^ to 3 inches. General form oval, very con- 

 vex ; length to width 17 : 10 ; the head and tail nearly equal in length ; 

 the thorax about two-thirds the length of either, and strongly trilobate ; 

 the axal furrows are carried less than half way up the head, and but a 

 very little way into the tail : head a quarter of a sphere, and regularly 

 convex, not peculiarly gibbous behind, where it is divided into three 

 nearly equal parts by the slightly converging axal furrows ; these curve 

 out again opposite the eye. The eye is of moderate size, gently lunate, 

 placed much nearer the furrow than the outer edge, and fully its own 

 length or more from the posterior margin. There is no internal ridge 

 at the place of the neck furrow. Facial suture divergent above the eye, 

 and slightly so below it, so as to cut the margin beneath the most pro- 

 minent curve of that organ. Rostral shield shuttle-shaped, covered by 

 transverse, close, sharp lines, the lower edge a little produced and over- 

 hanging. Hypostome ? Thorax of 10 narrow segments, the axis 



strongly marked, convex, and subfusiform ; wider than the pleurae in 

 the forward segments, in the last only equal to them. The fulcrum is 

 •very near the axis in the first segment, and in the last one is placed less 

 than half way along the pleurae, which are sharpened anteriorly beyond 



[ii. ii.] 2 c 



