388 F. JR. Coirper Eeed—The Gemts Trimiclem. 



middle, but joining up again into one near the genal angles. 

 Occasionally a sixth row is developed inside. These three innermost 

 rows have the pits radially arranged, but the others show no distribution 

 into regular radial lines, being mostly alternate in successive rows. 



The rows and pits in the upper plate of the fringe correspond 

 with those on the lower, but in front of the glabella a radial alignment 

 of the pits is often noticeable. The increase in the number of the pits 

 in the outermost row takes place by intercalation at the sides and 

 somewhat irregularly, so that the pits are unevenly spaced. The pits 

 of the second and third rows are larger, subequal, hexagonal, and 

 alternate, except just in front of the glabella. The fourth, fifth, 

 and occasionally sixth rows are of successively smaller pits, and show 

 a decided radial arrangement nearly all the way round. These features 

 are clearly developed in the smallest specimens which I have seen, 

 one of which measures across the fringe only 7 mm. 



In specimens from Pwllheli the inferior plate has the concentric 

 ridge between the third and fourth rows nearly as strong as the girder 

 between the second and third, and the differentiation of the plate into 

 an outer and inner band is obscure. The pits in the four outer rows 

 are radially arranged, but those in the two outermost rows are smaller 

 and tend to lie in grooves in front. Towards the genal angles there 

 are some extra pits intercalated in the first row, more or less destroying 

 the radial lines, and the pits in the third row become larger. A fifth 

 row is inserted laterally alongside the cheeks. 



On the upper plate there are four rows of pits of equal size and 

 at equal distances apart, arranged in weak radial grooves, and a fifth 

 row is inserted laterally. The correspondence, therefore, in this form 

 between the upper and lower plate is complete, and the regularity of 

 the radial arrangement in weak grooves is a marked feature, dis- 

 tinguishing it from the Onny River specimens. 



In the specimens of T. coticentricus from Dinas Mowddy, which 

 Salter^ termed var. Caractaci, and McCoy ^ labelled T. ffibbifrons, the 

 lower plate has the same general characters as those from Pwllheli, 

 but in the upper plate a few smaller pits intercalated in the first row 

 interrupt the regularity of the radial arrangement of the four main 

 concentric rows. 



The sj^ecimens of T. co7icentncus from Allt y Anker and Rhiwargor 

 have a strong girder inside the two outermost rows on the lower 

 plate, while in the upper jilate there are regular radial grooves in 

 which lie the pits of the four concentric rows, and all the pits are of 

 equal size, except in a few cases in which those of the first row are 

 rather smaller. 



T. favus,^ which is marked by the unusual outline of the fringe, 

 has the pits in the outermost row of the lower plate more numerous, 

 rather smaller, and more closely placed than those in the second and 

 third rows. All tliese three rows have their pits increasing in size on 

 each side of the middle line, especially near the anterior lateral angles 



^ Salter, dec. vii, pi. vii, 13. 6. 

 2 McCoy, Brit. Pal. Foss., p. 145. 



^ Salter, Mem. Geol. Surv., vol. ii, pt. i, p. 350, pi. ix, fig. 3; vol. iii, 

 p. 517, pi. xiii, fig. 9 ; dec. vii, pi. vii, p. 6. 



