194 



SILURIAN TRILOBITES. 



The axis of the thorax is regularly convex ; it is divided from the pleurae, which are 

 narrower by one fourth than the axis, by straight and parallel furrows, sharp but not 

 deep, and continued a very little way into the tail. The fulcral points are placed rather 

 more than one third out, and at nearly equal distance in all the rings ; and the pleurae 

 are thence curved, rather than bent downward, and turn but little backward except in 

 the front rings. The tips seem to be squarish or obtuse, and are not recurved. 



Tail semicircular, not much sinuated on the front border, the broad axis being 

 scarcely arched forward ; and the facet beyond the fulcrum, following the line of the 

 plem-ae, forms a very obtuse angle with the straight portion (not abruptly cutting off the 

 angle, as in many species, — II. Thomsoni, for instance). The general convexity is slight 

 and very regular, the border not being abruptly decurved ; and the axis is obscure, except 

 for the broad obtuse depressions on the front border ; it does not form any axal lobe. 



Comparing this form with the typical //. crassicauda, as given by Volborth, we find the 

 head less gibbous, and more generally convex ; the eyes smaller, and placed further out ; 

 the fulcra of the pleurae at nearly equal distance from the axis in all the rings ; and the 

 tail proportionately longer, with the axis indistinct, except quite in front. 



Some of these differences become resemblances when we compare this form with 

 //. Dalmmii, which Volborth has critically distinguished. But the head of that species 

 is less convex than ours, and longer too — semi-elliptical rather than semicircular. The 

 transverse section of a rolled specimen of that species is more obtuse than in ours, and 

 less pointed at the ends. The cheeks bend more steeply down. The tail is longer 

 than in the British specimens, and has the axis well marked out, &c. 



I cannot think, therefore, I shall be wrong in distinguishing our fossil by the name of 

 W. H. Baily, Esq., whose useful work as Palaeontologist to the Irish Survey deserves full 

 recognition. Mr. Baily has sent me several undescribed forms, which will be referred to 

 under their proper genera. 



Locality. — Caradoc or Bala. Dunabrattin Head, Waterford. (Mus. Irish Geol. 

 Survey, Dublin.) 



ILL.ENUS (III.) Davisii, Salter. PI. XXIX, figs. 10—16. 



IbLyKNUS CRASsiCADDA, Sharpe. Quart. Geol. Journ., vol. iv, p. 149. 1848. 



— Davisii, Salter. Decades Geol. Survey, No. 2, pi. ii, 1849. 



— — M'Coy. Synopsis Woodw. Fossils, p. 1/1, pi. i G, fig. 36, 18.>2. 



— — Salter. Ibid., Appendix, p. iv, 1852. 



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



— — Id. Siluria, 1st edit., woodcut 29, fig. 2, 1854 ; 2nd edit., 



p. 223, Foss. 44, fig. 2, 1859. 



— — Id. Catalogue Mus. Geol. Survey, pp. 5, 19, 1865. 



— — Id. Memoirs Geol. Surv., vol. iii, pi. xviii, fig. 9, p. 317, 1865. 



