.-) 



6 BRTTTSTI CAMBRIAN TRILOBITES. 



lobes, but the number of these is smaller. In M. speciosus the segmentation of 

 the axis is faint and the lateral lobes are without furrows. In AI. punctatus the 

 axis is divided into many rings, but the lateral lobes are without ribs. 



Of foreign species the most closely related is M. dmvsoni, Hartt. 1 The tail, 

 indeed, seems to be .almost indistinguishable, and the ornamentation of the margin 

 of the head is very similar. The glabella, however, in M. dawso?ii, is represented 

 both by Walcott 2 and Matthew 3 as extending to the anterior margin ; in 

 .1/. sculptus it is not much more than half as long as the head. According to 

 Matthew, M. dawsoni has only two thoracic segments; M. sculpt us has at least 

 three. It is clear, therefore, that the two forms must be specifically distinct. 



Typos. — Both of Hicks's types are in the Sedgwick Museum, Cambridge, and 

 are re-figured on Plate III. 



Horizon cold Locality. — Harlech Series: near Nun's Well, St. David's; West 

 of Porth Clais, St. David's. 



5. Microdiscus punctatus, Salter. Plate III, figs. 11 — 17. 



186 i. Microdiscus punctatus, Salter, Quart. Journ. Geol. Soc, vol. xx, p. 237, pi. xiii, fig. 11 a, b, c. 



1878. Microdiscus punctatus, Whiteaves, Amer. Journ. Sci., ser. 3, vol. xvi, p. 225. 



1882. Microdiscus scanicus, Linnarsson, De undre Paradoxidoslagren vid Andraruni, p. 29, pi. iv, 



figs. 17, 18. 

 1882. Microdiscus eucentrus, Linnarsson, ibid., p. 30, pi. iv, figs. 19, 20. 



1884. Microdiscus punctatus, Walcott, Bull. U. S. Geol. Surv., no. 10, p. 24, pi. ii, figs. 1, 1 a — c. 

 188(3. Microdiscus punctatus, var. pulchellus, Matthew, Proc. and Trans. Roy. Soc. Can., vol iii, sect. 



4, p. 74, pi. vii, fig. 12 a, b, c. 

 1896. Microdiscus pulchellus, Matthew, Trans. N. Y. Acad. Sci., vol. xv, p. 242, pi. xvii, fig. 8 a— f. 

 1890. Microdiscus punctatus, Matthew, ibid., p. 244. 

 1902. Microdiscus scanicus, Gri'mwall, Bornholms Paradoxideslag, p. 79. 

 1902. Microdiscus scanicus, f. eucentra, Gronwall, ibid., p. 79, pi. i, fig. 20. 



Head semi-elliptical, marginate. Glabella narrow, about two thirds the length 

 of the head, cylindro-conical, with two slight lateral impressions on each side, 

 separated from the cheeks by deep axial furroAvs ; no definite neck-furrow ; 

 produced posteriorly into a strong occipital spine which rises upwards and 

 backwards at an angle of about 45°; obscure traces of basal lobes sometimes 

 visible. Cheeks very convex, less prominent than the glabella posteriorly, but 

 elevated above it in front ; divided in front of the glabella by a groove which is 

 narrow and but slightly impressed on the outer surface of the test, broad and deep 

 on the internal cast; a strong occipital furrow close to the posterior margin; 

 external surface of the test finely punctate, internal casts nearly smooth. Margin 



1 J. W. "Dawson's Acadian Geology, 2nd ed. (1868), p. 654, fig. 228. 



2 Bull. U. S. Geol. Surv., no. 10 (1884), p. 23, pi. ii, figs. 3, 3 a. 



■' Trans. N. Y. Acad. Sci., vol. xv (1896), p. 240, pi. xvii, fig. 5 a— e. 



