CHETRURUS TTNICUS. 145 



the posterior part of the pygidium, and several detached, nearly perfect head-shields 



and pygidia of identical characters, are known from the same locality, Balclatchie, 

 and provide us with the means of giving the above complete description of the 

 species. The head-shields figured by Nicholson and Etheridge (M., fasc. I, 1878, 

 p. 100, pi. vii, figs. 5, 6) as belonging to Cheirurus gelasinosus ? are found to belong 

 to C. (Nieszh.) unicus. 



A few remarks are necessary as to the generic position of this species. 

 The characters of the head-shield, the oval glabella without a stalk, and the 

 longitudinally furrowed simple pleura? of the thorax, are amply sufficient to separate 

 it from Staurocephalus, and to suggest its location in Eccoptocheile or NieszhowsMa. 

 The relations of these subgenera or genera have been discussed by me 1 elsewhere. 

 The view more recently expressed by Raymond 2 that Nieszlcoivshia is a senile 

 expression of Pseudosplmrexochtis does not commend itself to me. The characters 

 of the pygidium decide that this Girvan species should be placed in NieszhowsMa. 

 The peculiar oblique basal pair of glabellar furrows are an additional point in 

 favour of this reference, and the resemblance to the species of this subgenus 

 described from the Ordovician Stage C of the Baltic provinces is very striking. 8 

 The pygidial characters are closely similar; the hugely developed first pair of 

 pleura?, the much reduced non-furrowed second pair, the axis of two complete 

 rings, with the pointed triangular piece behind marked by an incomplete furrow 

 near its anterior end, are features clearly seen in N. variolaris, Linnarsson. 4 



With regard to British forms and the confusion connected with this species, 

 Wyville Thomson's description of his original type specimen is correct so far as it 

 goes. Other specimens collected and similarly named by him which I have 

 examined, make it clear that Salter's figured examples of the pleura? and pygidium 

 also belong to the same species, and the figures correctly indicate their characters. 

 But the head-shield attributed by Salter (op. fit., pi. vii, fig. 22) to this species does 

 not belong to it, as the complete individuals from Grirvan prove. Nicholson and 

 Etheridge (op. cit.) have likewise confused two species under this name ; their 

 description of Ch. (N.) unicus is therefore faulty, and only some of their figures are 

 of specimens really attributable to it (M, pi. viii, figs. 10, 11, non figs. 9, 12 — 1G). 

 Furthermore, the pygidium attached to the pleura? figured by Salter (op. cit.), and 

 to the complete individual figured by Nicholson and Etheridge (M, fig. 10), agrees 

 with the imperfect one from Penwliapple Glen, called by the MS. name Stanro- 

 cephalus niacin rent by these authors and figured by them (M, pi. viii, fig. 11). But, 

 on the other hand, the specimens of head-shields labelled 8. maclareni, MS., by 

 Wyville Thomson and presented by him to the Museum of Practical Geology, 



1 Reed, 'G-eol. Mag.' [4], vol. iii (1896), p. 162. 



2 Raymond, 'Ann. Carnegie Mus.,' vol. iii, no. 2 (1905), p. 374. 



s Schmidt, ' Eev. Ostbalt. Silur. Trilob ,' pt. i, pp. 179—188, pi. ix, figs. 1—16 ; pi. xi, figs. 25—28. 

 * Linnarsson, 'Vestergotl. Camb. Silur. Aflagr,' p. 60, pi. i, fig. 6. 



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