CEPHALASPIDiE. 19 



6. List of Species of Heterostraci, arranged in Order of their Occurrence. 



Lower Ludlow Beds. Scaphaspis Lndensis, Salter. 



Upper Ludlow Beds. „ „ „ 



DowNTON Sandstone. Scaphaspis truncatus, Huxley and Salter ; Cyathaspis Banksii, Huxley and 



Salter. 



Ludlow Bone-ced. Scaphaspis Ludensis ?, Salter ; Scaphaspis truncatus, Salter. 



Upper Silurian (Gallicia). Scaphaspis Knerii, Lankester. 



Lower Devonian (of the Eifel). Scaphaspis Dunensis, Rcemer. 



Cornstones (West of England). Scaphaspis Lloydii, k^&ss. ; Scaphaspis rectus, Lunkestev : Cyath- 

 aspis Symondsi, Lankester ; Pteraspis rostratus, Agassiz ; 

 Pteraspis Crouchii, Salter. 



Lower Old Red Sandstone (Forfarshire). Scaphaspis, sp., Pteraspis Mitchelli, Powrie. 



7. Descriptions of Genera and Species. 

 Genusl. — Scaphaspis, Lankester. British Assoc. Report, Trans. Sect., p. 58, 1864. 



Cephalaspis (Lloydii et Lewisii), Agassis. Poissons fossiles, vol. ii, p. 149, 1835. 

 Pteraspis (Lloydii et Lewisii), Kner. Haidinger's Naturw. Abhandl., vol. i, p. 159, 



1847. 



Derivation. — <TKa<pri, a boat; aairlg, a shield. 



Characters. — Scutum cephahcum simplex, ovale, elongatum ; postice aliquanto attenu- 

 atum et fere brevispinosum ; superficie striis et liris longitudinalibus ornata antice trans- 

 verse dispositis. 



General Bemarhs. — This genus includes all the Heterostraci with a simple oval or 

 elongate shield consisting of but one piece. It is the earliest to appear, occurring in the 

 Lower Ludlow beds of the Silurian system. Of its British species two are Silurian, two 

 Devonian. Two species have been described from Continental localities — one by Roemei 

 from the Laacher See, in the Eifel ; the other from Gallicia, by Kner. The beds in 

 which these occur may correspond either to our Lowest Old Red or Upper Silurian 

 deposits. These two foreign species exhibit the characters of the English species in an 

 exaggerated form. Scaphaspis Dunensis of Rcemer (p. 20), from the Eifel, groups with Sc. 

 Lloydii and Sc. rectus, resembling them in its smooth ovoid form. Sc. Knerii (p. 20), 

 from the Silurian beds of Gallicia, on the other hand, resembles much more closely our 

 own Upper-Silurian species, Sc. Ludensis and Sc. truncatus, in its angular lateral margins 

 and median ridge. 



