F. R. Cowjjer Reed — The Genus Trlnucleus. 389 



of the fringe, but they decrease again in size towards the genal angles. 

 Thus the largest pits are found near the frontal angles. A fourth 

 row of smaller pits folloAvs inside the third row, and two short rows 

 are inserted at the anterior angles of the cheeks. The pits in the 

 rows alternate at the sides, but are more or less radial in front. 



On the upper plate there is a similar development of pit?, and in the 

 front part of the fringe the radial arrangement of the pits of the four 

 concentric rows and their gradual increase in size to the lateral angles 

 are well seen, but further back the radial arrangement is lost and they 

 alternate. A fifth, sixth, and occasionally a seventh row, all of smaller 

 pits, are developed inside at the angles, as on the lower plate, and more 

 small ones at the genal angles, where the regularity of position is lost. 



Tlie Tyrone examples of Trinucleus, which have been referred to 

 varieties of T. concentricus by various authors since Portlock divided 

 them up into several species, show special features of interest and 

 some peculiar characters. In only a few cases is the lower plate of 

 the fringe known, and the association of upper and lower plates is 

 often uncertain. 



The specimen (now in Jermyn Street Museum) figured by Portlock^ 

 as intermediate between his T. elongatus and his 1\ Caractaci shows 

 the cast of the lower lamella of the fringe in position with the head- 

 shield. There is a single row of pits on the outer band outside the 

 girder ; a second row immediately inside the girder has its pits as 

 large and in the same radii as those of the first row, and continues 

 regularly back to the base of the genal spines. A third row is only 

 developed from the front of the cheeks backwards, and the pits 

 alternate. A fourth row of small pits is present at the sides only. 

 But in the slightly re-entrant angle between the projecting anterior 

 end of the glabella and the cheeks there are 3-4 partly fused 

 minute pits in 5-6 short radial lines, while at the genal expansions 

 there are, as usual, large irregularly distributed pits. 



The specimen figured by B. Smith ^ from Tyrone as T. Portlocki 

 (which according to Salter^ is a variety of T. concetitricus, including 

 Portlock's T. Caractaci and T. latus) is the cast of a lower plate and 

 has identical characters with the above. 



The upper plate of T. elongatus represented by Portlock (op. cit., 

 pi. Ib, fig. 7) shows strong radial sulci in front, holding three pits in 

 a line, but the sulci and regularity of arrangement are lost at the sides, 

 where the pits of the three concentric rows are alternate and are 

 reinforced by a fourth inner row, while at the genal expansions six or 

 seven short concentric rows of smaller pits are inserted. 



In T. PortlocM, as represented by numerous specimens from Tyrone, 

 the lower plate has only one row outside the girder, as in T, elongatus, 

 and as far as its other characters are known seems to be indistinguishable 

 from it. The upper plate of the fringe which was preserved in the type, 

 and from which its characters were summarized by Salter " punctis ad 

 frontem subradiatis et in ordines 3 contractis ", shows four concentric 

 rows of pits in well-marked regular radial lines, with only three rows 



^ Portlock, Geol. Kep. Londond., pi. iB, fig. 6. 



^ Fearnsides, Elles, & Smith, op. cit., pi. viii, fig. 1. 



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



