192 



SILCRIAX TRILOBITES. 



Suhgenv^ 4. — iLi.iNrs proper. 



Ill-EXUs (II.) Bailti, n. sp. PI. XXVIII, fig. 14. 



Compare with Illj:>us crassicauda, Wahlenberg. Nova Acta Reg. Soc. Sci. Upsal., 

 vol. viii, p. 27, tab. ii, figs. 5, 6, 1821 — not of Portlock nor M'Coy. 

 — also with Illj:n-us Dalmaxi, Volborth. Mem. Acad. Imp. Sc. Petersb., 

 vol. vi. No. 2, p. 13, tab. ii, figs. 7 — 13. //. crassicauda, Dalm. et 

 Auctorum, fide Yolborth, 1863. 



m. (77.) magnus, 4 nncias hngus, .i2f latus, elUpticus, ohtusissimus, profunde trilohus ; 

 caput gihhum miiltum caudd planiore majus, oculis distaniibus reirorsis ; angulisque 

 rofundatis ; thorax caudd brevior. Caput transvermm valde convexum, vix gibhum, 

 glabella angustd, sulcis axalibus brevibus tertias capitis efficientibus. GencB abrupte 

 declives. Oculi modici eininentes, longe a glabella positi, et dimidium longitudinis eorum a 

 margine postico distantes. Axis corporis convexus, parallelus, pleuris vix latior : his fulcro 

 tenus {ad dimidium posito) planis, dein valde dejlexis, paulld {posticis pracipue) recurvis. 

 Cauda semicirculata [angulis oblique truncatis), subplana seu lente convexa, axe lato, 

 inter foveas latas tantiim conspicuo, sulcis axalibus nullis : margo acutus, nec re- 

 curvus. 



I have purposely confined the diagnosis and description to our single British speci- 

 men. It may, indeed, possibly prove identical with the common Scandinavian form above 

 quoted. But it is much larger than //. crassicauda, at least if the specimens which reach 

 England are fair samples of that species. (I have here figured 77. crassicauda and 

 its ally II. Dalrnani, for compaiison.) And while in the position and size of the 



Fig. 49. Fig. 50. 



b. 



Internal cast of lUttnus crtmicauda, Wahl. lUeenug Dalrnani, Volb. (After Volborth), Lower Silurian, North Rnssia. 

 showing alimentary canal (fuU of a. The caudal fascia, seen when the surface of the cast is broken away, 



ingested matter ?), from Volborth.' b. Interior of cast, showing lobes (attachments of muscles .'). 



^ Volborth "On the Smooth-tailed Russian Trilobites," 'Mem. Imp. Acad. Sc. Petersburg,' 7th 

 ser., torn, vi, No. 2, 1863. From this important and philosophical memoir our woodcuts illustrative of 

 the structure are all taken. Dr. Volborth discusses all the accessible points of the anatomy of Illaenus. He 

 regard.* the internal tube (fig. -19) as the dorsal heart, comparing it with that of Apus cancri/ormis. I 

 trust I am not presumptuous in supposing that the viscus represented by Dr. Volborth as a heart or 



