ILLyENUS. 



181 



Fig. 44. 



a. Labrum of Illanus cenirotus (after Volborth.) 



b. Labrum of lUanus crcusicauda (ib.). 



Illanus is as compact a genus as any in the whole Trilobite Order. It comprehends 

 almost as many distinct, if less varied groups, as Phacops itself ; and all of these have the 

 common facies of an inflated head and caudal shield, without external lobes or rings, and 

 that peculiar hemispheric contour to each, which is so marked a character of the whole 

 genus. Sometimes, the angles of the head are shortly spinous {Bysplanus), but usually 

 they are rounded off and unarmed [lUanus, Bumastus) ; sometimes the eye is subcentral 

 (77. centrotus), but more commonly it is placed behind the middle of the head, and near 

 the posterior margin (//. crassicauda, II. Boicmanni, ^"c). A few have the axal fun-ows 

 reaching quite up the head (//. distmdus, Barr.), but 

 in the great majority they reach less than two-thirds 

 this distance — and are often less than half-way. In 

 a very few the eyes are large (//. ocularis), but 

 usually they are small. The labrum varies in shape 

 in the different subgenera (see figs. 44 a, b). 



Lastly, the number of body-rings varies.^ Two 

 sections, perhaps distinct genera, Panderia and 

 Octillanus, have but 8 rings. Bysplams, including species with both spinous and 

 rounded head-angles, has only 9 rings. Blanus proper has 10 body-rings^ and this 

 includes many well-known species ; while Bumastus, Murchison, which, like Illanus, has 

 10 rings, has the axis very wide, and so little distinguished from the pleurae as to give 

 a unique aspect to the fossil. 



I agree with Barrande,^ that it is wiser, in the present state of our knowledge, to 

 include all these subgenera under one common name; dividing them into groups, 

 which may, by and bye, if the characters prove constant, be called genera. All of them 

 appear to agree in the following characters : 



Head hemispherical, or rather quarter-spherical, with 

 rounded contour, no marginal furrows, and a very slight 

 indication of neck-furrow. Glabella lobeless externally ; 

 within it has often 4 pairs of lobes, and is separated by in- 

 complete axal furrows from the tumid cheeks. Eyes lateral, 

 remote. Epistome striate, with a well-defined transverse 

 rostral shield, and bearing a somewhat triangular tumid 

 labrum, which has a strong border and expanded base (see 

 fig. 44 and PI. XXVII, fig. 7). 



Body-segments S — 10 ; usually 10, without grooves to 

 the pleurae. Tail lai-ge, hemispheric, with a short incom- 

 plete axis, showing numerous rings, but only within the crust. 



The genus, world-wide in its distribution, is neatly dis- 

 tinguished from all others by habit, as well as a combination of characters. These are 



Fig. 45. 



General type of Illmnus. 

 II. centrotus, Dalm. North Russia. 

 (After Volborth). 



1 Barraode has enumerated 1 1 genera in which the number of rings is variable. 



