152 ProGceclings of the Royal Physical Society. 



which should be exercised in accepting proportional measure- 

 ments as specific characters in the case of palaeozoic fishes ; 

 how different specimens of the very same species may be 

 distorted, squeezed up, or lengthened out into forms ap- 

 parently the most diverse ; and how that different modes of 

 preservation, different degrees of perfection of specimens, 

 afford room for the occurrence of fallacies in observation, the 

 avoidance of which requires all the acuteness which a prac- 

 tised eye can muster. Hence it is to be feared that some of 

 M'Coy's genera, and many of his species of Ganoids at least, 

 must fall to the ground. For instance, regarding the genus 

 Triplopterus, said to differ from Osteolepis by having only one 

 dorsal fin, I can, from examination of the original specimen, 

 fully confirm the suspicion expressed by Pander that it is 

 only an Osteolepis compressed from above downwards so as 

 to exhibit both ventral fins, one of which was interpreted by 

 M'Coy as the single dorsal and its fellow as the opposed 

 anal fin ! 



To M'Coy we owe, however, the separation of the true 

 Cephalasyidce from the other fishes, Pterichthys and Coccos- 

 teus, with which Agassiz had associated them, and the estab- 

 lishment of the latter as a group by themselves under the 

 name of Placodermata. And to him we also owe the term 

 " diphycercal," applied to that form of fish-tail in which the 

 vertebral axis is, as in the heterocercal form, gradually at- 

 tenuated, but runs straight backwards instead of turning up, 

 and the fin-rays being developed equally, or nearly so, above 

 and below, a more or less rhombic and symmetrical form of 

 caudal fin is produced. 



Eegarding the diphycercal tail of Diplopterus, he remarks, — 

 '' Those who think the theory of ' progressive development ' 

 worth refuting, may be glad to find that some of the oldest 

 known perfect remains of fishes have not exclusively hetero- 

 cercal embryonic types of tail as was hitherto supposed." 

 Here, in the first place, we are, I think, justified in altogether 

 objecting to the spirit in which this remark is made. It is the 

 truth, and nothing but the truth, which the true man of science 

 is " glad to find," not merely the corroboration of some pet 

 preconceived idea of his own. In the second place, our author 



