Structure and Classification of the Asterolepida3. 493 



Asterolepis, Eicliwald, with Pterichthjs, Agassiz ; and as the 

 priority lay with Asterolepis, he proposed to abolish the latter 

 name altogether, as being a mere synonym. We have also 

 seen that 'the attempt to base a generic distinction on a sup- 

 posed difference in the mode of articulation of the arms cannot 

 hold good, as Egerton's " thoracic " plates exist no more in 

 the one case than in the other. 



There is certainly a very remarkable resemblance in tlie 

 form and arrangement of the plates of the head and of the 

 arms, though as regards the former I must make a fe\y re- 

 marks. I have never in Pterichtki/s found any trace of the 

 " OS terminale " figured by Pander in his restoration of Aste- 

 rolepis, and concerning which he admitted that he had never 

 found it perfect in the Old Red Sandstone of Livonia; yet 

 its existence in the Russian form seems probable enough it, 

 as described by Pander, the anterior margin of the premedian 

 shows a sutural surface indicating the apposition of another 

 plate in front of it. I have seen nothing like the " as da-^ 

 Mum " in Pterichthys, though it may be the central part of 

 an arrangement like that which closes up the " orbit " in 

 Bothriolepis. Lastly, although tliereis in Pterichthys an 

 "angular" element in the same position as that shown in 

 Pander's figure of Asterolepis, it does not seem to project 

 backwards in the same way from the margin of the cephalic 

 shield. 



As the plates of the Russian Asterolepis have hitherto been 

 found only in a disjointed condition, it is natural that no tail 

 should have occurred in apposition with the body ; Pander 

 has, however, referred to the dermal covering of this part 

 certain curious bodies found in the Old Red of Russia, and 

 with which he considered the fragments known as Psammo- 

 lepis, Ag., Cheirolepis splendens and unilateralis, Eichw., 

 Microlepis exilis and lepidus, Eichw., and Ctenacanthus serru- 

 latus, Ag., to be identical. I have never had the opportu- 

 nity of examining any of these bodies, and can only say that, 

 judging from Pander's descriptions and figures, there does not 

 seem to me to be any reason for connecting them with Astero- 

 lepis, especially as he himself admitted that they ditfer in 

 structure from the body-plates, being composed of vaso-den- 

 tine, while the latter are composed of true bone. It is there- 

 fore clear that no comparison can be instituted between Pter- 

 ichthys and Asterolepis so far as the tail is concerned. 



There remains the body-carapace. This is more depressed 

 than in Pterichthys, but the number and general arrangenient 

 of the plates are the same. As regards their mode of articu- 

 lation Pander does not enter into any great detail either in 



