48 TRILOBITES OF GIRVAN. 



present in the Silurian form ; the axis is also narrower, more conical and more 

 pointed behind, and the third pair of pleura are longer. 



The pygidium of Sph. angustifroiis, Ang., does not appear to have been described, 

 unless the Keisley Spli. latirngatiis, Reed,^ belongs to this species, and it occurs on 

 a higher stratigraphical horizon (Stage F) than the above described Balclatchie 

 form. But the head-shields seem to be identical, and Billings' Sph. parvus- from 

 the Chazy Limestone scarcely suggests specific separation. 



Horizon and Localiti/. — Balclatchie Group (Llandeilo) : Balclatchie. 



Genus STAUROCEPHALUS, Barrande. 

 Stauroceplialus globiceps (Portlock). Plate VITT, fig. 3. 



1906. Staurocephalns glohiceps (Portlock), Reed, op. cit., pt. iii, p. 152. 



In my previous reference to this species doubt was expressed as to the correct 

 identification of the specimen recorded by Nicholson and Etheridge from Ardmillan. 

 But two nearly complete examples have now been found in the Balclatchie Beds 

 of Dow Hill, showing all the typical characters of the head-shield, thorax and 

 pygidium. Though the glabella is so different, the thorax and pygidium are 

 extraordinarily like those of Sphserocoryplie thomsoni,^ and indeed it is difiicult to 

 point to any constant distinctive features. Salter's''* woodcut of a Scottish specimen 

 gives a very fairly correct representation of the species. 



Family Phacopidjj:. 

 Genus PHACOPS, Emmrich. 

 Phacops (Chasmops) bisseti, Reed. Plate VIII, fig. 4. 



1906. Phacops {Chasmops) bisseti, Eeed, op. cit., pt. iii, p. 157, pi. xx, figs. 1 — 3. 



A fine specimen of a complete individual of this rare species has now been 

 obtained by Mrs. Gray from the Starfish Bed, and it is of special interest because 

 it has the hypostome (previously unknown) attached and in position. The full 



1 Eeed, ' Quart. Journ. Geol. Soc.,' vol. Iii (1896), p. 423, pi. xx, fig. 12. 



2 Raymond, 'Ann. Carnegie Museum,' vol. iii, no. 2 (1905), p. 372, pi. xiv, fig. 22; also 

 ' 7tli Ann. Rep. Vermont Geol. Surv.,' 1910, p. 246, pi. xxxvi, fig. 22. 



3 Eeed, ' Girvan Trilob.,' p. 146, pi. xviii, fig. 17; pi. xix, figs. 1 — 7. 

 * Salter, ' Mou. Brit. Trilob.,' p. 86, woodcut fig. 18. 



