Structure and Classification of the Asterolepidse. 33 



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 

 supposed difference in the mode of articulation of the arms 

 cannot liold good, as Egerton's "thoracic" plates exist no 

 more in tlie one case than in the other. 



There is certainly a very remarkable resemblance in the 

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

 arms, though as regards the former I nmst make a few 

 remarks. I have never in Ptcrichthys 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 Eed Sandstone of Livonia ; yet 

 its existence in the Eussian form seems probable enough if, 

 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 " os 

 duhmm " in Pterichthys, though it may be the central part 

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

 Bothriolepis. Lastly, although there is in Pterichthys an 

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

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

 backwards in the same way from the margin of the cephalic 

 shield. 



As the plates of the Eussian 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 Eed of Eussia, and 

 with which he considered the fragments known as Psamrno- 

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

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

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

 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 

 Asterolepis, especially as he himself admitted that they difter 

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

 dentine, while the latter are composed of true bone. It is 

 VOL. X. c 



