F. B. Cowper Reed — The Genun Triiiucleiis. 393 



In all these specimens of T. seticornis and T. Bucklandi the division 

 of the fringe into an outer and inner band (which has been shown to 

 be well marked on the lower plate by the girder in nearly every 

 species, but is not similarly defined on the upper plate) is indicated 

 by the marginal portion of the upper plate being horizontal and the 

 inner portion moi^e or less convex and steeply inclined so as to form 

 a kind of roll under the front of the glabella and the cheeks dying 

 out at the genal expansions. This roll, of course, corresponds to 

 the concave inner band of the lower plate. Barrande ^ described and 

 figured this feature in the Bohemian examples of T. Bucklandi, but in 

 immature individuals he remarked that the differentiation was not 

 developed.^ Salter^ stated that "the fringe is always steeply bent 

 down and follows the declivity of the cheek without any change of 

 direction, except in some specimens a gentle convexity ". 



The above I'emarks on the variation in the characters of the fringe 

 in British examples of T. seticornis are not intended to suggest 

 a classification based on these features alone. Any scheme of grouping 

 together the various forms of this species must be based, if possible, 

 on an aggregate of characters of all parts of the trilobite, and not on 

 the fringe alone, although frequently we have to depend only on the 

 liead-shield. When the peculiarities of the glabella and cheeks have 

 been discussed we shall be in a better position to make such an 

 attempt. But the foregoing somewhat detailed account of differences 

 in the fringe has been given to illustrate the amount of agreement or 

 want of similarity which may exist between the upper and lower 

 surfaces of the fringe of the same form. The general conclusion from 

 these observations points to a fairly general and complete corre- 

 spondence in the number of pits and rows on the two plates, but to 

 frequent dissimilarity in the matter of radial sulci. 



With regard to the connexion established by means of corre- 

 sponding pits between the upper and lower surfaces in T. seticornis, 

 it appears to be the case that there is generally no uninterrupted 

 communication by means of the small pits on the roll and on the 

 inner band, these pits being usually imperforate and merely 

 indentations in the surface and substance of the plates; but the 

 marginal pits and those at the genal expansions are perforated or 

 connected by short tubes, as in T. concentricus from the Onny River. 



Imperfectly hnoivn species. — There are several other British species 

 which have been desciibed and whose names have found their way 

 into geological literature, but with regard to our knowledge of their 

 fringe much is to be desired. In most cases figures have been given 

 of the head-shield exhibiting only the characters of the upper surface 

 of the fringe. Such are the following forms: — 



(1) The species of Trinucleus from the Whitehouse Beds of Girvan 

 described as Trinucleus sp. h by me in 1903^ is now better known, 

 and with regard to the fringe it shows on the upper surface two rows 

 of radially arranged large pits which in front tend to sink into short 



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



" Ibid., pi. XXX, figs. 14-16. 



^ Salter, dec. vii, pi. vii, p. 7. 



■• Reed, Mon. Girvan Trilob., pt. i, p. 14, pi. ii, figs. 8, 8« ; snppl., p. 160. 



