352 F. R. Cowper Reed — Notes on Trinucleu's. 



with modifications in other species, such as Tr. seticornis, Tr. Buck- 

 landi, Tr. Murchisoni, etc. 



In the case of Tr. Murchisoni, Salter,^ from the British Arenig rocks 

 (PI. XXVin, Figs. 4, 4ff), the incipient differentiation of the stalk 

 of the glabella is rather less advanced. The glabella as a whole 

 is of an ovoid form with the anterior part more inflated than 

 the posterior, but the pseudo-frontal lobe sinks down more 

 gradually into the stalk, which is not so independently cylindrical 

 as in Tr. fimhriatus. The first lateral furrows are very short 

 deep notches arising from the axial furrows and indent the sides 

 of the pseudo-frontal lobe at about half its length, but they are 

 not connected with the second pair by any groove or depression 

 as in Tr. fimhriatus. The second pair are large deep subcircular 

 pits completely isolated from the axial furrows, and lying at 

 the base of the pseudo-frontal lobe and inside the line of the first 

 pair of furrows. The third pair of large broad crescentic furrows 

 begin anteriorly in pits and curve back to the meso-occipital furrow so 

 as to embrace, as in Tr. fimhriatus^ a basal ring less strongly marked 

 than in that species, ending in a faintly impressed fourth pair of pits 

 in the meso-occipital furrow. The lateral composite lobes are readily 

 distinguishable, particularly in impressions of the surface of the head- 

 shield, and extend back as depressed sub-lanceolate areas from the 

 first lateral furrows to the base of the glabella, but they are less 

 swollen and distinct than in 2r . fimhriatus . 



The axial furrows start anteriorly in well-marked pseudo-antennary 

 pits, become less deep and wider behind the first lateral furrows, and 

 curve in gently posteriorly. JS^o sub-marginal pits or notches seem 

 to be present at their posterior ends as in some species, but their 

 position is marked by the size of the meso-occipital ring and its 

 lateral extension outside the longitudinal line of the lateral 

 glabellar pits. 



In Tr. £theridgei, Hicks, ^ the characters of the glabella and farrows 

 appear to be closely similar. 



Tr. Gihbsi, Salter,^ is probably allied, but none of the specimens 

 which I have examined allow the lobation of the glabella to be as 

 completely determined as in the species above described, and Salter's 

 figure is diagrammatic. 



We pass now to Tr. seticornis, auctt., using the specific name in the 

 broad sense common in this country. Specimens from the Sholeshook 

 (PI. XXVIII, Figs. 5, 5a), Slade, and Redhill Beds of Pembrokeshire 

 and from the Ashgillian of the North of England show a well- 

 differentiated swollen pseudo-frontal lobe, with the first furrows 

 represented by shallow small pits at its sides close to the axial 

 furrows and at about half its length. The second pair of segmental 

 furrows consist of large deep completely isolated pits at the base 

 of the pseudo-frontal lobe. The third pair are, as usual, in the same 

 axial line as the second pair and are crescentic in form, beginning 

 anteriorly in pits and ending, behind in a fourth pair of pits in the 



■^ Salter, Mem. Gaol. Surv., vol. iii, p. 51-5, pi. xiB, fig. 4. 



^ Hicks, Quart. Journ. Geol. Soc, vol. xxxi, p. 182, pi. ix, fig. 6, 1875. 



^ Salter, Mem. Geol. Surv., vol. iii, p. 516, pi. xii, fig. 10. 



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