Dr. R. H. Traquair — Devonian Fishes of Canada. 19 



of a continuous ring of quadrangular plates as they do in recent 

 Birds and Lizards and in extinct Ichthyosauria and Stegocephala. 



The plate labelled " preoperculum " by Whiteaves consists in 

 reality of three pieces, as in Bhizodopsis, Megalichthys, etc., of which 

 only one, the narrow semilunar posterior division (see my figure of 

 the head of Rhizodopsis), 1 can be supposed to represent a preoper- 

 culum, the other two being cheek-plates. Here I must confess to 

 have also failed to observe this in my previous study of Tristi- 

 chopterus, 2 as a re-examination of the same specimens incontestably 

 proves the presence of the same elements in that genus. 



The mandible shows very distinct evidence of being composed of the 

 same elements as in Bhizodopsis and Bhizodus. The laniary teeth are 

 like those of BMzodus in miniature, being rounded and fluted at the 

 base, smooth and two-edged higher up. A transverse section of the 

 fluted portion shows that the dentine is here thrown into complex laby- 

 rinthine foldings. I can see none but small teeth on the maxilla, as 

 in other Rhizodonts and Osteolepids, but the vomers are, as usual, 

 armed with strong tusks. Mr. Whiteaves states that " in Eustheno- 

 pteron, as in Tristichopterus, no trace can be detected of an azygos 

 jugular plate, or of any small lateral plates." There is certainly no 

 median jugular, but in one of our specimens the presence of five 

 narrow lateral jugulars is very distinctly shown. No specimen of 

 Tristichopterus as yet known is well enough preserved in this region 

 to show these small lateral plates. 



A very great interest attaches to the internal skeleton of the paired 

 fins, which in the case of the pectoral is tolerably accurately figured 

 by Mr. Whiteaves, though he does not seem very decided as to the 

 interpretation to be placed upon the arrangement shown. There is 

 a segmented axis consisting of four bony segments or mesomeres, 

 placed end to end, each being slightly constricted in the middle. Each 

 of these, except the last, has likewise attached to it on the preaxial 

 side of its distal extremity, a small bone, also constricted in the 

 middle and passing outwards and backwards at an acute angle ; 

 these small bones being clearly parameres. The first mesomere, aud 

 in one case also the third, gives off likewise on its postaxial side a flat 

 backwardly directed process, which is certainly only a process and not 

 a distinct bone. I may here also state that there is some evidence 

 of an additional mesomere in advance of or proximally situated to 

 that which I have termed the first, and bearing no paramere. A 

 very similar arrangement is found in the pelvic fin, whose internal 

 skeleton was not perfectly preserved in Mr. Whiteaves's specimens ; 

 here I find at least two mesomeres, each bearing a paramere, there 

 being, I think, also a probability of the presence of a third or distal 

 mesomere. 



The arrangement seen in the pectoral fin of Tristichopterus, which 

 I figured in 1874, 3 is essentially the same as that described above, 

 though at the time I wrote my description of the last-mentioned 

 genus, and taking the condition of preservation of the specimen into 



1 Trans. Roy. Soc. Edin. vol. xxx. 1881. 



2 Trans. Eoy. Soc. Edin. vol. xxvii. 1874. 



3 Trans. Roy. Soc. Edin. pi. xxxii. fig. 9. 





