348 F. R. Coirper Reed — The Genus Trinucleus. 



In the British forms referred to T. seticornis, His., or T. BticMandi, 

 l^arr., those specimens which have the shell of the fringe preserved 

 fail also to show a horizontal plane of fission traversing the tubes 

 which likewise form continuous hollow pillars connecting the upper 

 and lower lamellae separated by an intermediate space. The same 

 feature is observable in T, Nicliolsoni, Reed. Probably the per- 

 foration and union of the upper and lower pits into com[)lete tubes 

 are of a secondary nature, and we may thus account for the 

 obliteration of the median horizontal interlamellar space. Oehlert 

 (op. cit.) does not consider that the lamellae forming the upper and 

 lower plates of the fringe were really completely perforated, or that 

 direct communication by continuous canals was ever established ; for 

 he holds that a chitinous layer and conjunctive tissue existed on the 

 inner side of each lamella, preventing a free passage. But the 

 unbroken continuity of the tubes formed by corresponding pits can be 

 distinctly observed without a shadow of doubt in transverse sections 

 of the fringe of several British species, e.g. T. stibradiattts, Reed, and 

 it is particularly clear near the outer margin in this species and in 

 those cases in which the lamellae are nearly or quite in contact. 



The duplicate nature of the fringe was indirectly recognized by 

 Salter^ in the case of T. Lloydi, Murchison, for he states the fringe is 

 hollow in this species; though he had previously- put forward the 

 ■ incongruous view that the fringe originated hj the lateral fusion 

 of radial spines round the front of the head-shield. The presence of 

 an internal cavity in T. Lloydi may, therefore, have been conjectured 

 by him as due to the secondary hollowing out of a solid plate. 

 Eouault,^ though suggesting that the interlamellar cavity in the 

 biconvex fringe of T. Pongerardi, Kouault, may have acted as a float, 

 did not express the view that it resulted from the internal excavation 

 of a simple thick plate. 



If the fringe can be regarded as of the nature of an expanded border 

 (upper lamella) and doublure (lower lamella) in a head-shield with 

 a marginal suture, we should expect to find the inner edge of the 

 doublure, i.e. the lower lamella or plate, free ; and such is seen to be 

 the case in well-preserved specimens of Trinudeus. 



"Whether a fusion of the two lamellae has taken place in those cases 

 in which the fringe seems solid and simple (e.g. T. suhradiatus) is 

 difficult to decide, as the shell is mostlj" unknown, but it may be 

 regarded as probable. In other cases the lamellae may have been 

 so closely pressed together as to practically form one plate without 

 definite fusion. 



Beecher's view that the lower plate of the fringe represents the 

 conjoint free cheeks of more highly specialized Trilobites need not he 

 here discussed. At any rate, extensive modification in their structure 

 must have subsequently taken place, if this theory is correct, and it is 

 advisable to postpone the consideration of the homology and origin of 

 the fringe till we have completed our study of the genus. 



1 Salter, Dec. Geol. Surv., No. vii, pi. vii, fig. 3, 1853. 

 ■•* Salter, Proc. Geol. Soc, vol. iii, p. 251, text-figs., 1847. 

 ^ Renault, Bull. Soc. Geol. France, ser. ii, vol. iv, p. 311, pi. iii, figs. 1,. 

 la-c, 1846. 



