PHACOPS BISSETI 159 



In P. macroiira, Sjogr., Angelin 1 describes eighteen ribs to the pygidium, and in 

 the English form Salter states there are sixteen. The glabella, moreover, is much 

 more nearly square in P. macroura, and the frontal lobe is longer and less trans- 

 versely expanded. Schmidt- considers it probable that Salter's P. macroura really 

 belongs to P. eichwaldi, Schmidt, but the latter is described as having only 12 — 15 

 segments in the pygidium, which is also pointed and a little broader than long ; 

 the surface of the head-shield is, moreover, only finely granulated or quite smooth 

 instead of tuberculated, and the frontal lobe of the glabella measures less in 

 breadth than does the head-shield in length. P. eichwaldi therefore seems not only 

 distinct from Salter's P. macroura but also from the Girvan form. 3 



The general proportions of the head-shield of P. conicophthalmus (Boeck), and 

 the size and shape of the glabellar lobes with the overhanging lateral angles of the 

 frontal lobe, closely resemble the Girvan specimen, but the genal angles are only 

 furnished with short spines, and the eyes are smaller. The pygidium attributed 

 to this species by Salter (pp. cit., pi. vi, fig. 25) is completely different. 



The species P. ivesenbergensis, Schmidt, 4 from Stage E, has many points of similarity 

 to the Grirvan form, especially in the glabella and its lobes, and in their proportions. 

 But the " cat's ear " lobes are rather smaller, the first lateral furrows are rather 

 more oblique, the eye has only twenty-four rows of lenses with 10 — 11 lenses in 

 the middle row, the genal spines are not vertical, the pygidium is relatively rather 

 longer, and the axis appears to taper less rapidly. 



The form described by the author as P. marri 5 from the Coniston Limestone 

 may also be compared, but the shape and proportions of the glabella and lobes are 

 different. 



The head-shield in Mrs. Gray's collection has lost the free cheeks, and belongs 

 to a smaller and probably younger individual, which may account for the slight 

 differences noticeable in it when compared with Mr. Bisset's specimen. The 

 glabella has a less transverse appearance, the axial furrows diverging only at 

 70° — 75° instead of 75° — 80°. The inner angle of the "cat's ear" lobes of the 

 glabella is nearly 90° instead of 70°, while the posterior angle is 70° instead of 90°. 

 In all other respects there is complete agreement. The dimensions are as follows : 

 length of head-shield 1V5 mm.; length of glabella 9*5 mm.; basal width of 

 glabella 5 - mm. ; frontal width of glabella 12*5 mm. 



There is a fragment of a pygidium, probably belonging to the same species as 

 Mr. Bisset's specimen, from the Starfish Bed in Mrs. Gray's collection ; it shows 

 15 — 16 rings on the axis, the last 3 or 4 of which are indistinct, and 11 — 13 pleurae 

 on the lateral lobes. 



1 Augelin, 'Pal. Scaud.,' p. 9. 



2 Schmidt, ' Eev. Ostbalt, Silur. TYilob.,' pt. i, p. 117. 



3 Reed, < Geol. Mag.' [5], vol. i (1904), p. 386, pi. xii, fig. 3. 



4 Schmidt, op. cit., p. 115, pi. iv, figs. 10—12; pi. v, figs. 1—7; pi. x, fig. 20. 



5 Reed, 'Geol. Mag.' [4], vol. i (1894), p. 241, pi. vii, figs. 1—3. 



