386 F. B. Coicpev Reed — The Genus l^rinucleiis. 



T. Murchisoni ^ lias the outer band of the lower surface furnished with 

 primary radial sulci holding three pits, and shorter marginal secondary 

 radial sulci are intercalated regulaily between them and in front hold 

 two pits but laterally only one pit. On the upper surface there is 

 a corresponding number of primary and secondary sulci, but the 

 primary ones hold four pits and the secondary ones two pits. These 

 sulci on both surfaces extend right back to the genal angles. The 

 inner band of the lower surface bears close behind the strong ' girder ' 

 ill front a single concentric row of much larger pits agreeing in number 

 and position only with the primary sulci; laterally towards the genal 

 angles this single row splits into two rows of minute pits. It appears 

 that the fourth or innermost row of pits in the radial sulci on the 

 upper surface corresponds with this anteriorly single but posteriorly 

 double row of large pits on the lower surface (PI. XIX, Figs. 3, 3«). 



In specimens of T. JEtheridgei,- Hicks, from Scolton there is a very 

 similar development of pits in the fringe to that in 2\ Murchisoni. The 

 outer band of the lower surface bears in front primary ladial sulci with 

 three small pits in each, and between each pair of primary sulci there 

 is a shorter marginal sulcus intercalated holding two pits as in 

 T. Murchisoni; but laterally there is an absence of these shorter sulci, 

 and the primary sulci nearly die out, while the pits in them become 

 larger and more isolated. Near the genal angles a few of the shorter 

 intercalated radial sulci reappear. On the upper surface there are 

 similar primary and secondary radial sulci, the former holding 3-4 

 pits and the latter 1-2 pits. The sulci tend to die out posteriorly and 

 the pits become more isolated, those of the first row becoming larger 

 than the others. The short secondary radial sulci are wedged in on 

 the margin in such a way as to seem to branch off from the side of 

 the primaries in a Y-shaped manner, but they do not seem to originate 

 from a true bifurcation of the primaries as Hicks' figures indicated. 

 The inner band of the lower surface bears only one concentric row of 

 large isolated pits fewer in number than the sulci, with a few smaller 

 additional ones inside near the genal angles. 



None of the specimens of the interesting species T. Gihhsi., Salter, 

 which I have seen sliow the characters of the lower surface distinctly, 

 and Salter ^ did not describe it. Ent as far as is known it seems to 

 agree with the upper surface, which is wider in front than at the 

 sides or posteriorly, and bears radial grooves holding 3-4 pits; these 

 grooves decrease in length laterally as the fringe narrows, the pits 

 in them being reduced to two and ultimately to one. There is onlj' 

 a small expansion of the fringe at the genal angles, with a few 

 additional pores inside the main sulci. 



In T. suhradiatus^ the pits in the two rows on the outer band of 

 the lower surface are of equal size at the sides and alternate in the 

 rows, but the pits are smaller in front in the second row ; they are 

 larger and deeper than those on the broad inner band, which decrease 

 in size and depth to the inner edge. These small pits on the inner 



' Salter, Mem. Geol. Surv., vol. iii, p. 51.5, pi. xiB, fig. 4. 



- Hicks, Quart. .Jonrn. Geol. Soc, vol. xxxi, p. 182, pi. ix, fig. 6, 1875. 



' Salter, Mem. Geol. Surv., vol. iii, p. 516, pi. xii, fig. 10. 



* Eeed, Men. Girvan Trilob. (Palfeont. Soc), pt. i, p. 12, pi. ii, figs. 1-6. 



