32 BRITISH CAMBRIAN TRILOBITBS. 



with lour strong ridges between them, and possibly Salter may have counted the 

 number of ridges. Examination with a microscope, however, shows that th c 

 dividing lines between (lie head, the thoracic segments, and the tail lie along 

 these ridges, and that (he first and last grooves belong to the head and tail 

 respectively. 



According to Matthew, 1 the number of thoracic segments varies from two 

 to four. 



1. Microdiscus lobatus (Hall). Plate III, figs. 4 — G. 



1847. Agnostus hiatus, Hall, Pal. New York, vol. i, p. 258, pi. lxvii, fig. 5 a—f. 



1858. Agnostos lobatus, Rogers, Geol. Surv. Pennsylvania, vol. ii, p. 820, fig. 014 (1 — 4). 



1873. Microdiscus lobatus, Ford, Amer. Journ. Sci., ser. 3, vol. vi, p. 135, footnote. 



1886. Microdiscus lobatus, Walcott, Bull. IT. S. Geol. Surv., no. 30, p. 156, pi. xvi, figs. 1, 1</, 1 b. 



1888. Microdiscus lobatus, Shaler and Foerste, Bull. Mus. Coinp. Zool. Harvard, vol. xvi, p. 36, pi. ii, 



fig. 13. 



L890. Microdiscus lobatus, Walcott, Tenth Ann. Rep. U. S. Geol. Surv., p. 632, pi. lxxxi, figs. 4, 4a, 46. 



Head forming rather more than a semicircle, with a broad margin. Glabella 

 nearly cylindrical, rounded in front, elevated above the cheeks, reaching forward 

 to the wide marginal groove ; occipital furrow deep, sometimes two weaker trans- 

 verse furrows dividing the glabella in front of the occipital furrow into three 

 approximately equal lobes ; occipital ring strong, elevated in the middle into an 

 ill-defined tubercle. Cheeks small, semilunar, convex. Margin very broad in front 

 of the glabella, narrowing towards the sides, expanded again at the genal angles, 

 continuous with the flattened space behind the cheeks ; consisting, in front, of a 

 broad groove with an external raised rim, the rim dying out towards the side of 

 the head, where the margin becomes flat; slightly upturned again at the genal 

 angle ; in the anterior groove a row of small puncta3 is sometimes indistinctly 

 visible. 



Head-shield 1*5 mm. long, 2 mm. Avide. 



The cylindrical glabella and broad margin distinguish this form very easily 

 from all the other known British species. It was discovered by Mr. E. S. 

 Cobbold in an exposure of limestone 200 yards south of Comley Quarry, and it is 

 to him that I am indebted for bringing it to my notice. Although he examined a 

 large amount of material, and obtained some ten or twelve specimens, they all 

 appear to be heads. According to Walcott, the axis of the tail in 71/. lobatus is 

 marked by three well-defined transverse furrows, and the lateral lobes by three 

 principal furrows and three slight furrows (presumably intermediate). 



There appears to be a considerable amount of variation in the American 

 1 Trans. N. Y. Acad. Sci., vol xv (1896), p. 235. 



