ILLtENUS. 



197 



Comparing other species of the same sub-genus with ours, we find that //. crassicauda 

 is not only a much more convex form, but has the head rather gibbous behind ; the eyes 

 more remote and further back ; the tail scarcely longer than the thorax, the latter 

 having the fulcrum more remote in the front rings, and placed halfway out in the hinder 

 ones ; the incurved under portion or fascia (for example see fig. 50) far broader and less 

 concave {i. e. less reflected), and the surface sculptured by strong sharp lines ; these 

 lines are certainly not conspicuous in //. Davisii, though the latter is not quite smooth. 



From //. Portlockii its much greater convexity and the approximate eyes readily 

 distinguish it. The semicircular, not long tail, and oblique pleural tips distinguish it 

 from the species we have named //. Bosenhergii. II. Murchisoni does not need com- 

 parison ; and the position of the eyes will at once separate our neat regularly convex fossil 

 from the //. Bailyi. I hardly know any species that has better characters of habit ; and 

 it is rather strange it should ever have been confounded with the Swedish forms. 



Localities.- — Caradoc or Bala limestone and slate, west and north of Bala Lake, 

 at Rhiwlas chiefly ; also Pont-y-Glyn, DiS'wys, Corwen, and other intermediate places ; 

 Llanvvddyn, Montgomeryshire. (Mus. P. Geol. and Woodwardian Mus.) 



In Scotland, — the Wrae limestone ; and at Biggar, Peeblesshire. (Mus. P. Geol.) 



Ill^nus Portlockii, Salter. PI. XXVI, figs. 3, 4. 



Ill^enus crassicauda, Portlock. Geol. Rep. Tyrone, &c., pi. x, figs. 7, 8, 1843. 



— Portlockii, Salter. Decades Geol. Surv., 2, pi. ii, p. 3, 1849. 



— — Id. Siluria, 2ncl edit., Appendix, p. .539, 1859. 



— — Id. Morris's Catal., 2nd ed., p. 110, 1854. 



— — Id. Catal. Mus. Pract. Geol., p. 5, 1865. 



11. {III.) modicus, 2^ uncias longus, late ovatus, depresms, thorace cauddque ejusdem 

 longitudinis fere. Caput convexum [injuniore] glabella angustd, sulcis axalihus hrevihm ; 

 oculis distantibus posticis. Thoracis axis pleuris longe latior est^ his fulcra tenus planis, 

 dein subito valdeque reflexis, deflexis fulcro distante. Cauda planata, transversa, oblonga, 

 subquadrata, angulis externis longe truncatis ; axe magno trientem cauda efjiciente et 

 ultra, sulcis axaJibus subparallelis. Fascia lata concava. 



A species well figured in Gen. Portlock's very unfortunate plates ; but he referred it, 

 apparently without much consideration, to the Illanus crassicauda of Dalman, from which 

 it difiers in every particular except the number of body-rings, a sub-generic character. 

 It is of a flattened shape, with short wide triangular tail, and with pleurae more abruptly 

 bent down than in any other species. II. PortlocMi is, perhaps, one of the most distinct 

 of the British Illani. It is only yet known in one locality, the famous Caradoc schists 

 of Desertcreat parish, Co. Tyrone ; a spot rendered classic by Portlock's excellent work. 



