350 F. R. Coivper Reed — Notes on Trinucleiis. 



pair, and it is only the second and third pairs to which reference 

 is usually made. In all the best-known British species there are 

 three pairs present, though they are in various stages of development 

 or obsolescence, the first pair indenting the sides of the so-called 

 ' frontal lobe ', which is therefore not homologous with the true frontal 

 lobe of other genera (e.g. Cheirurus) in which the first lateral furrows 

 bound the frontal lobe posteriorly. For this reason the ovoid or 

 sub-hemispherical swollen anterior portion of the glabella in those 

 species of Trinucleus with a stalk or neck to the glabella may be 

 termed the pseudo-frontal lobe. 



Another character which has been usually neglected in describing 

 the head-shield is the true course and behaviour of the axial furrows. 

 For it is not usually made clear in descriptions of the glabella that 

 the longitudinal depressions or furrows bounding the stalk of the 

 glabella on each side are not the posterior continuations of the axial 

 furrows but different structures of the nature of the longitudinal 

 furrows bounding the median lobe in some species of Lichas and 

 Acidaspis, and that there is a more or less definite elongate composite 

 lateral lobe between the stalk and the true axial furrows, as in some 

 species of Ampyx. The presence of these composite lateral lobes 

 has generally been overlooked, and the stalk has been regarded as 

 representing the whole posterior width of the glabella, in spite of 

 the meso-occipital ring extending further out on each side at its base. 



Certain other pits associated with the axial and other cranidial 

 furrows have been insufiiciently noticed in the past, and the fact that 

 the segmental furrows are mostly reduced to pits situated at the inner 

 ends of obsolescent or obsolete furrows has led to confusion in the 

 homology of the five or six pits which are frequently present on each 

 side of the glabella. 



The reduction of the segmental furrows to mere isolated pits on the 

 surface of the glabella is by no means uncommon in various genera of 

 different orders and families of trilobites (e.g. Bronteus, Bro7iteopsis, 

 Oryctocephalus, and some species of Acidaspis, Babnanites, and Cyhele), 

 leading to the more or less complete longitudinal fusion of the lateral 

 lobes of the glabella and the formation of composite lateral lobes as 

 sometimes in Lichas. 



In the genus Trinucleus the British species which shows the nature 

 and number of the segmental furrows and the course of the axial 

 furrows better than any other is Tr. fimlriatus, Murchison, from 

 the Llandeilo of Builth (PL XXVIII, Figs. 1-3). McCoy ^ 

 described and figured this species from Pencerrig, Builth, and showed 

 in his figure the posterior course of the true axial furrows (though 

 not quite correctly), and the presence of the elongated composite 

 lateral lobes on each side of the neck, but he does not comment on 

 this peculiarity, and only mentions the occurrence of two lateral 

 furrows on each side. A study of a large series of specimens from 

 this locality allows of a fuller description of the head-shield of this 

 interesting species. The anterior half of the glabella is swollen and 

 sub-pyriform, forming the pseudo-frontal lobe ; the posterior half is 

 depressed rather suddenly behind it. There is a pair of small, short 



^ McCoy, Syn. Paleeoz. Foss. Woodw. Mus., 1855, p. 146, pi. iE, fig. 16. 



