Ostracodermi. 



77 



posterior median spine, an anterior lateral pair of plates pierced 

 by the eyes, and a posterior lateral pair pierced by openings 

 from the gill-chamber. There is a single ventral shield, and 

 the tail in Pteraspis is also proved to have been scaly. All the 

 Heterostracan shields are ornamented externally with very fine 



FIG. 102. Restoration of shield of Pteraspis rostrata, Ag.,upper aspect (after Lankester). 

 Lower Old Bed Sandstone ; Herefordshire. 



concentric lines of vascular dentine. They are typically Lower 

 Devonian, and the largest specimens scarcely ever exceed six 

 inches in length. 



So far as known, the Osteostraci are almost confined to the 

 Uppermost Silurian and the Lower Devonian rocks, only one 

 specimen having been found in the Upper Devonian (Canada). 

 The typical genus is Cephalaspis (Fig. 103), represented in the Table . cases 

 Collection by the finest known specimens from Forfarshire and Nos. A, B. 

 Herefordshire. Special attention may b3 directed to the unique 

 group of Cephalaspis Murchisoni from the Lower Old Red Sand- 

 stone Passage Beds of Ledbury, Herefordshire, presented by 

 George H. Piper, Esq., F.Gr.S. The head-shield is rounded or 

 tapering in front, truncated behind, and the eyes appear close 

 together in the middle ; its outer tuberculated layer is usually 



FIG. 103. Cephalaspi 



Murchisoni, Egert. ; L. Old Red Sandstone (Passage Beds), Ledbury, 

 Herefordshire. 



removed, and then the middle layer is exposed with its coarse 

 network of blood-vessels. A pair of small nippers at the back 

 of the shield are probably not fins, but connected with the 

 aeration of the gills. The body is covered with rings of bony 

 scutes ; there is one dorsal fin ; and the tail is distinctly hetero- 

 cercal. In Auchenaspis and Didymaspis some of the body-rings 

 are fused together immediately behind the head-shield. Trema- 

 taspis is a remarkable allied genns. -^^^ 



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