ILL^NUS. 



195 



B. {II.) minor, 2| uncias lonpis, ellipticus, obtusiis, convexus, bene trilohus ; capite 

 et pygidio ejusdem magnitudinis fere, ocuUs parvis approximatis. Caput semiovatum, 

 convexum nec gibbicm, glabella angiistd sulcis axalibus brevibus convergentibus. Gena declives, 

 anguUs qiiadratis. Oculi parvi, elongati, glabella propinqui, d basi capitis diametrum 

 eoriim distantes. Thorax minus convexus, fusiformis, sulcis axalibus minime profundis. 

 Plevrm fulcra axi propinqua, rectce, angusta. Cauda semicirculata fere, margine antico 

 vix sinuato, angidis vix truncatis ; axe lato, inter sulcos latos {in pullo conspicuos) paul- 

 lulum elevate ; margine deflexo. Fascia lata, striis crebris. 



One of the many peculiar fossils which abound in the more argillaceous parts of the 

 Bala Limestone, but Avhich give place to others directly the matrix becomes sandy. A list 

 of such species is given in the third volume of the Memoirs of the Survey, quoted 

 above.^ This difference led Prof. Sedgwick and myself, in 1844, to believe there were 

 tw'o bands of limestone ; but the researches of the Geological Survey have shown 

 that there is but one principal band. Indeed, the calcareous bed called the Bala 

 Limestone is continuous, or at least intermittent, over something like 4000 square miles. 

 From Kildare, in Ireland, to the Grug limestone^ at Llandeilo, in South Wales, in one 

 direction ; thence to Bala, in North Wales ; at Horderly, in Shropshire ; as the Coniston 

 band in Westmoreland ; in Ayr, Peebles, and probably through all the Western High- 

 lands, this remarkable band of limestone ranges, and keeps the same fossils throughout, 

 with mere local variations, such as I have just noticed and, perhaps, there is no species 

 of Trilobite so characteristic of this band (it is not the most abundant) as Illcenus Davisii. 

 II. Bowmanni nearly everywhere accompanies it, but that species has a wider range, and 

 seems to have been necessarily less confined to one kind of sea-bottom. The reader will 

 pardon this digression for the sake of the facts ; the Bala Limestone being the key to the 

 geology of the Lower Silurian (Middle Bala group, Sedgwick). 



//. Davisii is a small species, seldom more than two or two and a half inches long, 

 convex, but not gibbous, wtII trilobed, but not wath deep axal furrows, and with the 

 margin rather suddenly bent do-wn. The eyes are forward and approximate, as com- 



' I may, perhaps, not have so good an opportunity of showing this change which takes place in one 

 and the same bed of limestone, at a distance of certainly not two miles. Where the Bala Limestone has a 

 muddy matrix the Trilobites are as in the first column : where it is sandy, the other group is conspicuous. 

 Argillaceous Bala Limestone. Arenaceous Bala Limestone. 



Trinucleus seticornis. Trinucleus concentricus. 



Illsenus Davisii. Illsenus Bowmanni. 



Cheirurus bimucronatus. Phacops apiculatus. 



Asaphus radiatus. Asaphus Powisii. 



Agnostus trinodus. Calymene senaria 



Ampyx tumidus. (Beyrichia complicata). 



The shells follow the same rule, but less strictly.— J. W. S., 18.53; 'Mem. Geol. Surv.,' vol. iii, p. 2/3. 

 2 This Grug limestone is often confounded with the Llandeilo limestone : it is brought close to it by 

 faults, but has quite different fossils. 



