350 F. R. Coicper Reed — The Ge)ius Trinncleus. 



as distinct from the addition to the number of pits in the same row 

 has so far been always traced to the division of one of the pits, and 

 the new row starts oif from the inner half of such a transversely 

 bisected pit (PI. XVIII, Figs. 5, 11). Such is the origin of the second 

 row in tiie case of T. Bucklandi and of the extra rows of small pits 

 on the 'roll' of the fringe of T. seticornis. Pits arising in this way 

 by fission lie in the same radial lines as those of the originating 

 row, but those developed by intercalation in the same row disturb 

 the regular radial arrangement and lead to alternation in the pits 

 of successive rows or to irregularity in their arrangement. 



The base of the glabella is not always the centre of the circle to 

 which the radial lines of pits in the fringe converge. The point of 

 convergence may be situated further forward, and towards the genal 

 angles the lines may therefore slope back, giving a fan-like arrange- 

 ment. If the pits are not in sunken grooves this shifting forward of 

 the centre leads to the appearance of alternation of pits in successive 

 rows as the genal angles are approached (e.g. T. Lloydi). 



With regard to the pits on the expansions of the fringe at the genal 

 angles or posterior outer angles of the cheeks, there seems to be no 

 definite rule ; in all the species they are added and interpolated 

 irregularly, and even in the same species there is considerable variation 

 in their number and position which is not entirely due to differences 

 of age. 



4. Differences in the Upper and Loiver Surfaces of the Fringe. 



The upper and lower surfaces of the fringe in many species of 

 'Trinncleus show a considerable difference in the development and 

 arrangement of the pits, as Salter long ago noticed. The concentric 

 rows on the two surfaces may or may not correspond even in number, 

 and radial grooves may only be developed on one surface and not on 

 the other or in different parts of the two surfaces. The aspect of the 

 two surfaces is therefore frequently different, and this is especially 

 noticeable in the case of impressions of the surface. In most specific 

 descriptions, however, only the characters of the upper surface are 

 recorded, and unfortunately this is occasionally unavoidable owing to 

 our imperfect knowledge of the lower surface. 



Aa. Lower Surface of Fringe. 

 With regard to the lower surface of the inferior lamella or plate of 

 the fringe, we may first note that it is rarely either uniformly flat or 

 gently convex or weakly concave as is the upper lamella. In nearly 

 every case it is more or less angulated at one of the concentric ridges 

 some distance inwards from the outer edge. This angulation may be 

 feebly marked, particularly near the genal angles (e.g. T. Goldfussi, 

 Barrande), or it maybe quite absent {T. Pongerardi'), but usually traces 

 of it are discernible. Where it is strongly developed the division of 

 the lower surface into an outer more or less horizontal flat band and 

 into an inner inclined flat or more generally concave band is at 

 once noticeable. Usually these bands are of unequal width, the 

 outer one being generally of regular and smaller width and the inner 

 one variable but widening considerably towards the genal angles and 



