76 



Ostracodermi. 



fossils are exclusively Palaeozoic, and were first discovered in 

 the Cambro- Silurian and Devonian formations of Russia and 

 North America, 



Table-cases, 

 Nos. A, B, 

 and Wall- 

 case, No. 4. 



Table-case, 

 No. A. 



Ostracodermi. 



The armoured notochordal animals which possessed neither 

 lower jaw nor paired fins, and are hence now supposed to be 

 related to the lampreys, are found only in Upper Silurian and 

 Devonian rocks. They were for a long time erroneously 

 classified with Coccosteus and its allies (see p. 96) under the 

 name of " Placodermata " (plate-skinned animals), but are now 

 distinctly separated from the latter and grouped under the 

 name of OSTRACODERMI (shell-skinned) or OSTRACOPHORI (shell- 

 bearers). The head and anterior portion of trunk are covered 

 with plates ; while there are remains of median fins, and often 

 also scales, in the caudal region. No traces of the internal 

 skeleton have been found, and the notochord must have been 

 persistent. 



There are three groups cf Ostracoderms distinguished by 

 the structure and arrangement of the anterior shield. Firstly, 

 there are the Heterostraci (anomalous-shells), comparatively 

 simple, of which the plates exhibit no bone-cells in their tissue 

 and comprise three superposed layers an inner " nacreous " 

 layer of lamellse, a relatively thick middle zone of polygonal 

 chambers, and an outer hard layer of vascular dentine. 

 Secondly, the Osteostraci (bony-shells), also with simple shields, 

 which exhibit bone-cells in part at least of their tissue. 

 Thirdly, the Antiarcha, of which the shields comprise many 

 symmetrically disposed plates, are provided with a pair 

 of lateral appendages, and likewise exhibit bone-cells in their 

 tissue. 



The earliest known fish-like shield belongs to Cyathaspis, a 

 member of the Heterostraci. One specimen has been recorded 

 from the Wenlock Limestone of the Island of Gothland in the 

 Baltic Sea, and a ventral shield (originally named Scaphaspis 

 ludensis) has been found in the Lower Ludlow of Leintwardine, 

 Shropshire. Cyathaspis Banksi occurs in the Upper Ludlow 

 " Bone-bed " and Downton Sandstone. The typical Heteros- 

 tracan genus, Pteraspis (Figs. 101, 102), has a dorsal shield com- 

 posed of seven pieces : a rostrum, a large median disc, a 



FIG. 101. Restoration of Pteraspis rostrata, Ag. Lower Old Red Sandstone ; Herefordshire. 



