F. R. Cowper Reed — Notes on Trinucleus. 353 



meso-occipital furrow, thus bounding laterally a more or less marked 

 basal ring. The stalk of the glabella is of a sub-cylindrical shape ; it 

 is well marked off from the pseudo-frontal lobe in front and from the 

 depressed almost obsolete narrow composite lateral lobes, which have 

 suffered such reduction as almost to escape notice, though they are 

 still more clearly traceable in some specimens than in others. The 

 posterior ends of the axial furrows, which have almost disappeared 

 behind the pseudo-frontal lobe, being lost in the wide depression 

 between the stalk of the glabella and the cheeks, are marked by 

 a strong pair of sub-marginal pits, lying very close to the posterior 

 edge of the head-shield, outside the axial line of the glabellar 

 pits and at the sides of but behind the meso-occipital ring. 

 Between successive thoracic segments there are similar pits in 

 the axial furrows. It may here be remarked that the fact that 

 the anterior width of the first and following rings of the 

 thoracic axis is considerably greater than that of the stalk of 

 the glabella indicates that this stalk does not really correspond 

 to the total width of the axial (i.e. glabellar) portion of the 

 head-shield. The sub-marginal intersegmental axial pits of the 

 thorax as well as the sub-marginal pits of the head-shield thus 

 lie in a longitudinal line outside the second and third pits of the 

 glabella. 



The pseudo-antennary pits in Tr. seticornis are always distinct and 

 in their typical position. 



The allied Tr. Bucklandi^ Barr.,' as represented by the smaller 

 specimens from Girvan ^ (PL XXVIII, Figs. 6, 6^ ; PI. XXIX, 

 Fig. 1) which appear to completely agree with Bohemian examples 

 and to differ slightly from the larger specimens from Girvan 

 attributed to the same species, has the pseudo-frontal lobe differentiated 

 as in Tr. seticornis, and the first pair of lateral furrows of the 

 glabella of the same character and in the same position. The second 

 pair of furrows are not, however, simply pits, but form short, deep, 

 transversely oblique furrows, bounding the pseudo-frontal lobe behind, 

 but they do not arise quite from the axial furrows, though very near 

 them, and they extend about one-third of the way across the width of the 

 glabella on each side, deepening internally and ending abruptly. The 

 third pair of furrows are also not simply isolated pits on the surface 

 of the glabella, but are short, transverse, broad furrows arising 

 nearly from the axial furrows with their inner ends deepest ; they 

 are also directed obliquely forward to the middle, and are only feebly 

 connected behind with the deep fourth pair of pits in the meso-occipital 

 furrow. The single lateral composite lobe is thus almost segmented 

 and tends to be split up into imperfectly separated paired lateral lobes 

 owing to the length of the second and third pairs of furrows, but the 

 lobes are not clearly individualized. The lateral furrows are better 

 developed and less reduced in this species than in the others. Owing to 

 the weakness of the connexion between the third pair of furrows and 

 the fourth pits, the basal ring is obscurely defined, but the stalk of the 



^ Barrande, Syst. Silur. Boheme, vol. i, p. 621, pi. xxix, figs. 10-17. 

 " Nicholson & Etheridge, Men. Silur. Foss. Girvan, fasc. ii, p. 190, pi. xiii, 

 figs. 14, 15, 20 ; Reed, Girvan Trilobites, 1903, pt. i, p. 10, pi. i, fig. 11. 

 DECADE VI. — VOL. I. — NO. VIII. 23 



