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



ing them to be spinous, and not interspinous. It would indeed be 

 odd if so great a morphological difference existed between such 

 similar elements in such closely-allied fishes. Comparing specimens 

 of Eusthenopteron and Tristichopterus in the Edinburgh collection, there 

 can be no doubt as to their exceeding similarity of structure, though 

 the differences mentioned in reasons 3 and 4 seem to me to justify 

 the retention of the former generic name. It must be remembered, 

 however, that in such questions much depends on the idiosyncracy of 

 the individual writer — differences, which to some minds may seem 

 generic, are not always of the same importance in the eyes of others. 



Mr. Whiteaves describes the cranial buckler as divided by a trans- 

 verse suture into two unequal parts, an anterior or frontal, and a 

 posterior or parietal plate. This is of course true, but he makes no 

 mention of the constituent bones of these two great divisions of the 

 cranial shield, which can, in fact, only be made out when the shield 

 is viewed from the internal or non-sculptured aspect. The posterior 

 part may be seen in this way to consist of two elongated parietals, 

 each of which is flanked by two smaller plates, a squamosal behind 

 and a postfrontal in front, exactly as in Rhizodopsis, Osteolepis, etc. 

 Likewise the anterior portion consists of two frontals in contact with 

 each other in the middle line, but without the pineal foramen seen 

 in many Osteolepids (Osteolepis, Diplopterus, Thursius), while in 

 front of these, and received posteriorly in an angle between their 

 anterior extremities is an oblong median ethmoidal, flanked on each 

 side by a lateral ethmoidal, while the space between these and the 

 premaxillae is apparently covered by a number of small polygonal 

 plates as in Holoptychids, Ehizodonts, and Osteolepids generally. 

 I have remarked above that the sutures between these plates can 

 only be seen when the cranial shield is examined from the internal 

 aspect ; on the outside nothing is seen but a median groove, noticed 

 by "Whiteaves, separating the two frontals, and bifurcating in front 

 behind the median ethmoid. 



A circle of small quadrangular or slightly wedge-shaped plates 

 occurs in the position of the eye, of which six are seen in one 

 specimen before me, while in Mr. Whiteaves's paper five are 

 represented in his restored figure (pi. vi. of his second quarto 

 paper), and the whole circle, "about twenty-six," in the specimen 

 erroneously referred to Phaneropleuron (ib. pi. x. fig. 1). Of these 

 plates Mr. Whiteaves observes that they " are probably homologous 

 with the circumorbitals of Traquair's restoration of Dapedius, as well 

 as suggestive in a general way of the still more highly specialized 

 sclerotic plates in the eye of Ichthyosaurus and Megalosaurus." 



Now, as it is clear that these little plates cannot be both of two 

 such very different things at once, a choice must be made, and from 

 their shape and their position relative to the other bones in the 

 head in which I have observed them, I have no doubt that they are 

 sclerotic and not circumorbital in their nature. If this view be 

 correct, we have in Eusthenopteron a condition almost unique among 

 fishes, for though sclerotic ossifications are not uncommon amongst 

 them, in no fish except certain Coelacanths do they assume the form 



