PTERICHTHYS. 91 



part of the posterior ventro -lateral is in like manner considered to be a separate 

 plate, and designated " posterior ventral." 



As already mentioned, Pterichthys was considered by Pander to be synonymous 

 with Asterolepis, and it, consequently, does not come in for special treatment in 

 the " Placodermen." Nevertheless some very instructive figures of Scottish 

 specimens of this genus are given in pi. v of that work, and it is interesting to 

 note that he represents the posterior ventro-lateral plate as undivided, and 

 explains the apparent separation of a " thoracic " plate from the anterior ventro- 

 lateral as being due to fracture. 



I have already mentioned (p. 74) that the existence of the supposed " thoracic " 

 plate to which the pectoral limb in Pterichthys was articulated was looked upon 

 by Beyrich, Lahusen, and Zittel as constituting a character separating this genus 

 from Asterolepis. Pander was, however, right — there is no such separate plate 

 for bearing the pectoral limbs in Pterichthys, or in any other Asterolepid genus. 

 But a valid ground of distinction may be found in the mode of articulation of the 

 anterior median dorsal plate, which in Pterichthys is overlapped behind by the 

 posterior dorso-lateral, instead of overlapping it, as in Asterolepis, and I for my 

 part consider this character to be quite sufficient to warrant the separation of the 

 two genera. It is, however, not so with my friend Professor Jaekel of Berlin, who 

 in a recent paper l pronounces himself unwilling " auf Grund so unwesentlicher 

 Differenzen, wie der randlichen Verbindungsart zweier Rumpfplatten, Gattungen 

 auseinander zu halten." But there is still another difference between the two 

 genera of which I only became aware recently, on examining the text-figure on 

 p. 53 of Jaekel's paper quoted above. There we have a representation of the 

 plates of the ventral surface of " Asterolepis Milleri" the elements of the right 

 arm being shown. 



Now in Figs. 34 and 35, p. 65 of the present work, I have drawn the plates 

 of the pectoral limb of Pterichthys as they undoubtedly are in Asterolepis, the 

 forearm consisting of two centrals, dorsal and ventral, two pairs of marginals and 

 a terminal, the latter forming the pointed extremity of the limb. But in Jaekel's 

 figure, just quoted, the " terminal " is dismembered so as to produce not only 

 an additional ventral (presumably also an additional dorsal) central, but likewise 

 two additional marginals, the inner of which forms the point. That Jaekel is 

 right here I have convinced myself by a careful re-examination of specimens 

 in the British as well as in the Edinburgh Museum, so that we have again a 

 difference between the two genera which cannot be overlooked, and which I have 

 embodied in the present amended restoration of Pterichthys, Figs. 51 and 52. 



In these amended figures of mine another change must be noted. In my 



1 " Ueber die Organization und systematische Stellung der Asterolepiden," ' Mai-Protokoll der 

 Zeitschr. der deutschen G-eol. G-esellscli.,' Bd. It, 1H03, pp. 4d — 60. 



14 



