74 BRITISH CAMBRIAN TRILOBITES. 



than provisional. Most of their remains are very fragmentary. In many species 

 the head alone has been recognised, and in most the tail is quite unknown. It is, 

 moreover, often very difficult to decide whether the pleuras end in short points or 

 in long spines, for the ends are frequently concealed in the matrix. In many cases 

 it is equally difficult to say whether the margin of the tail was entire or was armed 

 with minute spines. 



Of the four genera belonging to this group, Leptoplastus, Sph&rophthalmus, and 

 Gtenopyge have all been found in England. Eurycare has not yet been definitely 

 recognised, but some imperfect and distorted specimens from Borth and elsewhere 

 may possibly belong to that genus. 



The generic characters of Sphdsrophthalmus appear to be as follows : 



General form decided^ convex, with strongly marked axial grooves. Head of 

 moderate width ; glabella reaching the anterior margin ; fixed cheeks rather narrow, 

 convex, very little expanded behind the eyes ; eyes placed very far back and near 

 to the glabella ; free cheeks with a rounded external margin from the middle of 

 which the cheek-spine springs abruptly. 



Thorax of about nine segments, very narrow posteriorly; axis wider than the 

 pleuras; pleurae bent downwards at the fulcrum, terminating in points which in 

 the posterior segments are produced into backwardly directed spines. 



Tail more or less triangular, with a wide axis and narrow lateral lobes; margin 

 entire. 



1. Sphaerophthalmus alatus (Boeck). Plate VIII, figs. 1 — 6. 



1838. Trihhites alatus, Boeck, Keilhau's Gsea Norv., p. 143. 



1848. Olenus humilis, Phillips, Mem. Geol. Surv., vol. ii, pt. 1, p. 55, figs. 4, 5, 6, p. 347. 



1854. ? Sphserophihalmus alatus, Angelin, Pal. Scand.,p. 49, pi. xxvi, fig. 9. 



1854. Olenus sphx-nopygus, Angelin, op. cit., p. 43, pi. xxv, fig. 3. 



1854. ? Anopocare pusillum (pars), Angelin, op. cit., p. 50, pi. xxvii, figs. 1, 2. 



1857. Sjohasrophtalmus alatus, Kjerulf, Geol. d. siidl. Norw., p. 284 (teste Brogger). 



1864. Olenus (Sphserophihalmus) humilis, Salter, Mem. Geol. Surv., Brit. Org. Remains., dec. xi, 

 pt. viii, p. 7, pi. viii, figs. 9 — 11. 



1865. Sphaerophtalmus alatus, Kjerulf, Veiviser ved geol. excursioner i Christiania omegn., p. 2. 



1866. Olenus (Sphieroplithalmus') humilis, Salter, Mem. Geol. Surv., vol. iii, p. 302, pi. v, fig. 12. 

 1871. Olenus humilis, Phillips, Geology of OxforJ, p. 68, fig. 8. 



1880. Sphxrophthalmus alatus, Linnarsson, Geol. Foren. Stockh. Forh., vol. v, p. 137, pi. v, 



figs. 6—10. 

 1882. Sphaerophtalmus alatus, Brogger, Die Silur. Etagen 2 und 3, p. 119, pi. ii, figs. 14, 14a. 

 1890. Sphxrophthalmus alatus, Pompecki, Die Tril. -Fauna i)reuss. Diluvialgesehiehe. Beitr. zur 



Naturk. Preussens, vii, p. 89, pi. iv, figs. 27, 27a. 

 1893. Sphierophtliahnus alatus var. canadensis, G. F. Matthew, Trans. Roy. Soc. Canada, vol. xi, Sect. 



iv, p. 107, pi. xvii, figs. 11a, b, 12a, b. 



Non 1866. O/euus (Spha ropMJtalmus) alahis, Salter, Mem. Geol, Surv.. vol. iii. p. 302, pi. iv, figs. 3, 3a. 



