TRINUCLEUS SUBRADIA1TS. 13 



less regular row. Inside these marginal rows and on the more steeply inclined 

 inner portion of the fringe are many more or less concentric rows of radially 

 arranged pits of much smaller size. Towards the posterior margin of the head- 

 shield these inner radial rows lose to some extent their regularity, and have 

 inserted between them and the marginal rows many irregularly placed pits of 

 intermediate size, occasionally with an obscure radial or linear arrangement. The 

 genal angles of the fringe are slightly extended behind the posterior margin of 

 the head-shield. Glabella pyriform, strongly elevated near the anterior end and 

 swollen, laterally compressed and posteriorly sharply carinated ; possesses no 

 distinct neck, but two pairs of short broad furrows indent the sides, and two 

 pairs of deep pits lie at the base of it in the axial furrows. The glabella rises 

 steeply from the inner side of the fringe and does not overhang or invade it. 

 Occipital furrow strong. Occipital ring broad in centre and produced backwards 

 into short triangular nuchal spine. Cheeks sharply marked off from fringe, 

 slightly swollen, much less elevated than glabella, as broad as long ; bear median 

 ocular tubercle connected with glabella by fine oblique ocular ridge. 



Axial furrows well marked, with a pair of pits in their course near front end of 

 glabella. Glabella and cheeks ornamented with closely set minute pits. 



Thorax too imperfect for description, but apparently showing the usual generic 

 characters. 



Pygidium unknown. 



Dimensions. — 



Length of head-shield . . . 7 — 8 mm. 



Width of ... 15—16 „ 



Affinities. — In outline the head-shield resembles T. favus, Salter (' Mem. Geol. 

 Surv.,' vol. ii, pt. 1, 1848, pi. ix, fig. 3). The carinated glabella recalls T. lloydi, 

 Murchison (' Silur. Syst.,' 183 ( .>, pi. xxiii, fig. 4), and so do the nuchal spine and 

 the radiating rows of pits on the fringe, but the Girvan form differs by the strong 

 differentiation in the size of the marginal and inner pits of the fringe, the more 

 numerous inner rows, the non-concave fringe, the lateral furrows of the glabella, 

 and the shape of the head-shield. 7'. radiatus, Murchison (op. cit., pi. xxiii, fig. 3), 

 bears a somewhat strong resemblance, but has the pits on the fringe lying in 

 furrows with a more distinct radial arrangement. T. concentricus, Eaton, resembles 

 it in the pear-like shape of the glabella and in the neck-spine, but the fringe is 

 completely different. The peculiar elevation and carination of the glabella are 

 found also in T. reticulatus, Ruedemann (' New York State Mus. Bull.,' No. 49, 

 1901, p. 41, pi. hi, figs. 15, 10, 18, 10), but this species has no neck-spine and an 

 entirely different fringe. It was from the erroneous identification of some of 

 these specimens that the species Tr. ornatus and Tr. concentricus seem to have 

 been recorded by the Geol. Survey as Girvan forms (' Mem. Geol. Surv., Silur. 

 Rocks Brit.,' vol. i, Scotland, pp. 071 — 08'.)). 



