CHEIRURUS THOMSONI. I 1 7 



nodular swelling and constricted transversely just beyond. Fulcrum situated at 

 rather less than half the length of pleura, Extra-fulcral portion of pleura 

 depressed, semicylindrical, curved backwards, and ending in free point. Facets 

 for articulation present on inner portion of pleura. The last body-ring appears 

 to be fused with the front of the pygidium in adult individuals, and to have its 

 pleuras rather broader and shorter. Surface of thorax granulated. 



Pygidium short, broad; central portion consisting of a wide, ill-defined, conical 

 axis, composed of three prominent rings and a small posterior piece. Axial furrows 

 nearly obsolete. One pair of huge pleura?, with broad bases embracing the whole 

 side of the axis, and forming the lateral lobes, produced backward into broad, 

 divergent spines more than twice the length of the pygidium. The base of these 

 pleura? extends along the whole lateral border of the pygidium, the posterior border 

 between them being simpl} r rounded and arched backward gently. There is no 

 second pair of pleuras. A faint median longitudinal groove is seen on the basal 

 portion of the pair of pleura?, and the more distal portion is weakly angulated 

 longitudinally. On both sides of the axis, in the axial furrows, are situated three 

 pairs of deep pits, corresponding with and at the ends of the intersegmental 

 furrows across the axis. Surface of pygidium (including spines) finely granulated. 



Remarks. — The head-shield of this species was figured by Salter (op. cit., pi. vii, 

 fig. 22) as belonging to Stauroceplialns unicus, Wyv. Thomson, but two crushed 

 head-shields from Piedmont Glen presented by Wyville Thomson to the Museum 

 of Practical Geology as examples of his MS. species Staurocephalus maclareni are 

 apparently identical. The pygidium, however, in the same collection, labelled 

 also by Wyville Thomson as belonging to the same species, has been shown above 

 not to belong to these head-shields, but to be really attributable to Ch. (Nieszh.) 

 miicus, sens. str. It is doubtful, therefore, on which specimen he founded his 

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

 1878, pi. viii, figs. 9, 12-16, non. figs. 10, 11) as Stanrocejphalus unicus are almost 

 identical with that figured by Salter. These authors also examined Wyville 

 Thomson's specimens mentioned above, and state that the two head-shields from 

 Piedmont Glen fully bear out Salter's view of the identity of S. unicus with 

 S. maclareni. It appears, therefore, desirable, in the presence of all this confusion, 

 to give a new name (Sph. thomsoni) to these head-shields, especially as we have 

 now found also the thorax and pygidium attached to the head in complete 

 individuals. The specimen of a pygidium from Shalloch Mill, figured in a wood- 

 cut by Nicholson and Etheridge (M, p. 120, woodcuts 6 A, B) as " Staurocephalus 

 ': aniens, possibly a variety of it, or a new species," is also referable to this species. 



There is a fairly distinct variety of this species from the Balclatchie Group, 

 the head-shield of which was figured by Nicholson and Etheridge, as above stated. 

 In this the globular portion of the glabella scarcely overhangs the front margin, 

 but is pressed back, so as almost to conceal the basal nodules (which are much 



