F. R. Coujjer Reed — The Genus Trinucleus. 347 



the British Isles. The following notes have been compiled from 

 a comparative study of this material supplemented by specimens from 

 the Jermyn Street Museum and elsewhere. The frequent addition of 

 specimens to the collection at Cambridge since Salter and McCoy's 

 time have allowed for a more comprehensive and complete survey to 

 be made than was then possible, thougli several doubtful points still 

 await better material before they can be cleared up. 



My attention was first directed several years ago to the inadequate 

 or imperfect descriptions of the fringe ; and accordingly the study of 

 the structure of this portion of the head-shield forms the first paper 

 m the series v^hich is intended subsequently to deal with the othei- 

 characters of the members of this genus, with a view to a revision 

 of the British species. 



I. The FfiiNGE. 



The structure of the ' fringe ' around the head-shield of Trinucleus 

 has been studied by many palaeontologists from time to time, but in 

 the case of the British species of the genus the investigation of its 

 characters has sometimes lacked precision owing to the poorness or 

 scarcity of the material available. Our knowledge, moreover, has 

 mainly been confined to the upper surface of the fringe, and the 

 features of its lower surface have very rarely been described. It is 

 remarkable that it has not been more noticed that the upper and lower 

 surfaces frequently show a considerable difference in the arrangement 

 or development of the pits, so that casts of the two surfaces are dis- 

 similar and lead to confusion in describing the specific characters 

 unless this fact is clearly recognized. 



From the examiuation of a large series of specimens of the principal 

 British species it has been found possible to supply to some extent 

 this gap in our knowledge, though some points in the structure and 

 relation of the various types remain obscure for the present. 



1. Double Nature of the Fringe. 

 With regard to the nature of the fringe itself, the presence of 

 a marginal suture in all the species seems to be now generally assumed,' 

 though its existence has not been always demonstrated, and in most 

 of the British species can rarely be directly observed. "We should 

 expect it to be most clearly noticeable in those forms in which the 

 fringe obviously consists of two lamellae separated by an intervening 

 space, as in the species from the Bala Beds of the Onny River, usually 

 now referred to T. concentricus, Eaton. The suture in these specimens 

 from Shropshire in which the shell is preserved is seen just above the 

 angular outer edge, as Oehlert ^ states is the case in T. Bureaui, Oehlert. 

 But the hourglass-shaped hollow pillars representing the opposite and 

 communicating pits on the upper and lower surfaces are continuous 

 and show no transverse plane of fission, though they break at the 

 waist when the fringe is horizontally split; the internal tubes are 

 also uninterrupted (PI. XVIII, Eigs. la, lb). 



^ Woods, article on Trilobita in Camb. Nat. Hist., vol. iv, ch. viii, p. 226, 

 1909. 



^ Oehlert, Bull. Soc. G^ol. France, ser. ni, t. xxiii, p. 317, text-figs, k and I. 



