F. R. Coxvper Reed — The Genus Trinudeus. 349 



2. Character of the Pits. 

 The nature and development of the pits which dot the upper and 

 lower surfaces of the fringe vary somewhat. Generally they are 

 merely conical pits of greater or less depth, the apices of opposite and 

 corresponding ones frequently touching one another and sometimes 

 communicating by a pore, thus forming hourglass-shaped structures 

 in a cross-section of the fringe. The pores when occurring at the 

 apex of each pit may connect by means of a special minute pit or short 

 tube, or the apices may be directly in contact. Occasionally the floor 

 of each pit is raised in the centre into a low rounded boss or mamelon, 

 the centre of which is depressed and perforated, and it is connected 

 with the corresponding pit on the opposite surface by a short 

 fine tube, as is beautifully shown in some casts of the so-called* 

 T. concentricus from the Onny Eiver (PL XVIII, Fig. 1). The same 

 structure is observable in the larger pits at the genal expansions in 

 most British specimens of T. seticornis, and in T. Buchlandi from 

 Girvan. Barrande ^ did not show this plainly in his figures of 

 T. Buchlandi from Bohemia. In his figures of T. ornatus (op. cit., 

 pi. xxix, figs. 7, 8), he illustrated the common type of pit in the 

 genus, consisting of a snbconical circular depression with a small 

 pore in the centre of the floor. 



3. Arrangement of the Pits. 



The pits of successive concentric rows are in some cases sunk in 

 radial grooves or may be fused more or less completely so as to form 

 radial sulci. In the most extreme cases (e.g. T. hihernicus, Reed^) the 

 pits completely lose their identity and the perforations then alone 

 indicate the number of pits which have amalgamated (PI. XYIII, 

 Fig. 2). All stages between the simple linear arrangement of pits in 

 radial lines and their depression into grooves and fusion can be traced ; 

 and the definiteness, straightness, and elevation of the dividing radial 

 ridges likewise vary from the extreme development of T. fimhriatus, 

 Murchison,^ to the scarcely differentiated structures in T. Nicholsoni.^ 

 Frequently different stages occur in the same species or in different 

 parts of the fringe of the same individual (e.g. T. Murchisoni, 

 Salter 5). 



The increase in the number of pits in the same concentric row is 

 brought about by intercalation, and this intercalation does not seem to 

 take place at any definite point in the row, judging from those cases 

 (e.g. T. concentricus from the Onny River) in which it has been 

 most clearly observed in the marginal (i.e. outermost) row (PI. X7III, 

 Figs. 4, 7, 12). 



On the other hand, the addition to the number of concentric rows 



^ Barrande, Syst. Silur. Boheme, vol. i, pi. xxix, figs. 10-17; pi. xxx, 

 figs. 14-16. 



- Eeed, Geol. Mag., Dec. IV, Vol. II, p. 52, Pi. Ill, Figs. 2-7, 1895. 



2 Murchison, Silur. Syst., t. xxiii, fig. 2 (head only) ; McCoy, Syn. Brit. Pal. 

 Foss. Woodw. Mus., p. 146, pi. iE, fig. 16. 



^ Eeed, Geol. Mag., Dec. V, Vol. VII, p. 212, PL XVI, 1910. 



^ Salter, Siluria, 2nd ed., p. 50, fig. 7 ; id., Mem. Geol. Surv., vol. iii, p. 515, 

 pi. xiB, fig. 4. 



