ILL^NUS. 



211 



sides slope evenly away to the slightly recurved margin, within which the broken speci- 

 men shows the narrow internal fascia. 



Locality. — Llandovery Sandstone (Lower Llandovery Rock) of Mullock, Girvan, 

 Ayrshire ; a locality first described by Murchison, and well searched by the per- 

 severing collector, the late Alexander MacCallum (see ' Quart. Geol. Journal,' vol. vii, 

 1851, p. 170), who discovered most of the species in his own district. Illcenus Thomsoni, 

 Trinudeus concentricus, and Atrypa hemisphcerica occur in the same deposit of hard 

 yellow sandstone and grit, but no Pentameri, such as are found in the overlying " May 

 Hill Sandstone" of Saugh Hill. 



Sub-yenus 6. — Ectill^nus, Salter, 1866. 



Ill^nus (EcTiLLiENUs) PEROVALis, Murchison. PI. XXVI, figs. 5 — 8. 



Ill^nus PEROVALIS, Murckison. Sil. Syst., p. 661, pi. xxlii, fig. 7, 1839. 



— — Salter. Siluria, 1st ed., pi. iv, figs. 13, 14, 1854. 2ud ed., 



pi. iv, figs. 13, 14, 1859. 



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



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



— — Id. Mem. Geol. Surv. iii, p. 256, 1866. 



//. {Edill.) late ovatus, obtusus, depressus, 2^ uncias longus, 2 uncias fere latus ; capite 

 semicirculato, quam cauda minore, sed thorace brevi pauUo longiore; oculis anticis submargi- 

 nalibus. Caput glabella angustd, haud tertias latitudinis ejus efficiente, sulcis awalibus brevibus 

 redis. Oculi {?) suturaque facialis omnino atitici submarginales ; anguli obtusi. Thorax bene 

 trilobus, axe angusto subfusiformi, pleuris angustiori. Fleurce ad fulcrum (circiter tertias 

 posito) plana, dein fracta, subrecta, apicibus obliquis. Cauda viargine antico siibrecto, 

 axe brevi conico. Fascia angustior, subconcava. 



This remarkable species, certainly the type of a very distinct sub-genus (probably a 

 new genus), was first obtained by Sir R. I. Murchison from the lower part of the Llandeilo 

 formation, in the Shelve country, west of the Stiper Stones. In the 3rd vol. of the Memoirs 

 of the Survey, I have inadvertently included it in the Arenig group, but wrongly so : see 

 above, p. 182. This I learned by a visit to St. David's, where Mr. Hicks, Mr. Lightbocly, 

 and myself obtained a good specimen from the lower part of the true Llandeilo rocks at 

 Abereiddy Bay, near Llanrian (fig. 8) ; and Mr. Thos. M'K. Hughes, of the Geological 

 Survey, had some time previously obtained a similar specimen (fig. 7). These are larg^i- 

 than Murchison's specimens in the Geological Society's collection, and show more clearly 

 what must be the true position of the eye, if there be one. With this Trilobite is found, 

 but rarely, the double Graptolite Bidymograpsus Murchisona, so that its place in the 

 series is unquestionable, for the Graptolite just mentioned is a typical Llandeilo fossil. 



We still want to know more about the head of this species, probably not a rare one ; 

 and the small free cheek has not yet been found. It must have been quite narrow ; very 



