F. R. Coivper Reed — The Genus Trinucieus. 351 



there corresponding to the genal expansions at the inner edge of the 

 upper lamella. The distribution and character of the pits on the 

 two bands have generally marked peculiarities, as will be mentioned 

 in the sequel. 



4h. The Outer and Inner Bands. 



The division into an outer and an inner band is well seen in 

 T. Murchisoni, T. firnhriatus, T. hilernieus, T. Buchlandi, T. seticornis, 

 T. JSficholsoni, and T. sulradiatus. 



The two portions or bands are separated as a rule not only by an 

 angular junction but also by the marked enlargement or thickening of 

 the concentric ridge situated along this line. This specially developed 

 ridge may be termed for convenience the 'girder', as it appears to 

 have been formed for strengthening purposes ; it is particularly well 

 seen in the British examples of T. seticornis and T. Bucklajidi, but 

 it occurs more or less strongly developed in all the species possessing 

 an outer and inner band to the lower plate. Salter ' briefly noticed 

 and figured it in T. Lloydi and in T. concentricus,'^ but Nicholson 

 and Etheridge, jun.,^ described it as a groove instead of a ridge in 

 the Girvan examples of T. Buchlandi, forming their ideas from 

 impressions of the lower plate. Bernard Smith ^ mentioned it also as 

 a groove in connexion with T. Portlochi, Salter, from Tyrone, but for 

 the most part this structure seems to have been ignored. The author 

 described it as a ridge in the case of T. NicholsoniJ' 



4c. The Positio7i of the Girder. 



The position of the girder in relation to the concentric rows of pits 

 on the lower surface varies to some extent in different species. In 

 T. Lloydi Salter showed the girder lying inside the two outermost 

 rows, and this is found to be the commonest position, and he described^ 

 also the British form of T. concentricus as having the girder similarly 

 placed, i.e. between the second and third rows. This may be well 

 seen in most Welsh examples from Bala Beds, but m those from 

 Pwllheli the girder is not clearly differentiated, the concentric ridge 

 between the third and fourth rows being nearly as strong as that 

 between the second and third rows. In T. concentricus from the 

 Onny Eiver it is interesting to find the girder readily recognizable by 

 reason of its greater strength inside the second row in the anterior 

 median part of the fringe, but posteriorly on each side it decreases in 

 size and is scarcely larger than the ridge inside the first row. 



In T. sulradiatus'' the first and second concentric ridges are of 

 equal or subequal size (though in front the ridge behind the first row 

 is most pronounced), the girder thus not being clearly differentiated ; 



1 Salter, Dee. Geol. Surv., No. vii, pi. vii, 1853. 



- Salter, Mem. Geol. Surv., vol. iii, p. 517, pi. xix, figs. 4, 4a. 



^ Nicholson & Etheridge, jun., Mon. Girvan Silur. Foss., fasc. ii, p. 192, 

 1882. 



■* Feamsides, Elles, & Smith, Proc. Koy. Irish Acad., vol. xxvi, sect. B, 

 No. 9, p. 122, 1907. 



5 Keed, Geol. Mag., Dec. V, Vol. VII, p. 212, 1910. 



" Salter, Mem. Geol. Surv., vol. iii, p. 517, pi. xix, figs. 4, 4a. 



^ Eeed, Mon. Girvan Trilob. (Palseont. Soc), pt. i, p. 12, pi. ii, figs. 2, 5, 6. 



