D)'. R. H. Traquair — On Brejjanaspis. 159 



membrane no abrupt line can be seen, though the scales tend to 

 diminish in size distally. And in a specimen ot" the recent Angel-fish 

 (Rhina sqnatina) now before me, I see no sudden change in the 

 character of the shagreen granules as they pass from the surface of 

 the body on to that of the broad pectoral fin. As regards evidence 

 of flexibility, the Silurian specimens of Coelolepidas are all crushed 

 absolutely flat, but in the case of the large Thelodiis Paget from the 

 Forfarshire Old Red, which has not been subjected to such an 

 extreme degree of compression, it does seem to me that there is 

 decided evidence of flexibility, as well as thiifiiess, at the posterior 

 external angles of the parts which I have supposed to represent fins. 



Secondly, Dr. Smith Woodward objects to the idea of " a motile 

 organ, originally used for balancing, if not for progression, losing its 

 function, not by atrophy, but by conversion into a rigid structure 

 continuous with its base of support," a phenomenon for which he has 

 " tried in vain to find a parallel in the animal kingdom." 



It is perhaps a bold idea, and 1 may be wrong, but let us look for 

 one moment at the case of the Asterolepids (Antiarcha of Cope). 

 Dr. Smith Woodward thinks it improbable that the pectoral members 

 of Pterichthjs are homologous with those of the ' Class Pisces,' but 

 there they are, organs which must have been used for progression, 

 enclosed in rigid plates, though provided with two joints, one at 

 the junction with the body and another about the middle of the 

 appendage itself. Dr. Smith Woodward maintains, however, that 

 the fixed pectoral spines o? Acanthaspis, which he considers to be an 

 Asterolepid or Antiarchan, are homologous with the moveable 'arms ' 

 of Asterolepis or Pterichthys. If he is right in this, would there not 

 be something of a parallel here, unless we believe that the fixed 

 spine has been the original condition from which the complex 

 moveable limb of Pterichthys has been evolved, which is not very 

 probable ? This, of course, is not an argument upon which I can 

 insist, seeing that I personally consider Acanthaspis to be, not an 

 Asterolepid, but a Coccostean, closely related to PhJydcenaspis, Traq., 

 the very similar pectoral spine of which Dr. Smith Woodward was 

 himself the first to observe.' 



It is, however, not the object of the present communication to 

 «nter into an exhaustive discussion of the disputable questions raised 

 in my ' Silurian ' Memoir and in Dr. Smith Woodward's comments 

 thereon, — that I must defer to another time and another place. My 

 purpose has been to complete the description of the remarkable 

 Heterostracan genus Drepanaspis, according to light thrown on the 

 subject by material acquired since the publication of the memoir to 

 which I have so often referred. 



^ In Coccosfeus itself the pectoral spiue is so small that in my description of the 

 skeleton of C. decipiens (Ann. Mag. Nat. Ilist., Feb., 1890) I described and figured 

 it merely as a process of the interlateral plate, from which it is, however, quite 

 <listinct. There certainly is a ' Bruststachel ' in Coccosfeus, but as it is not only very 

 small, but absolutely fixed, it can hardly be called a ' Ruderorgan.' 



