1848. | SIR P. EGERTON ON PTERICHTHYS. 31] 
would be more inclined to regard it as the hexagonal dorsal plate of 
the Pterichthys major. Besides the general form, the squamous 
suture shown on the right-hand side,—-of course taking into account 
the circumstance that it is a cast, or impression rather, not the plate 
itself, that is exhibited,—was evidently bevelled off from the under 
side of the plate, as is the case in the dorsal plate of the Pterichthys, 
not from its outer side, as would be the case were it the ventral plate 
of a Coccosteus. Agassiz’ genus Homothorax (tab. 31. fig. 6) has, I 
suspect, like his genus Pamphractus, no existence in nature. It 
seems to be founded on but a drawing of some imperfect specimen of 
Pterichthys of the Upper Old Red of Fife. But regarding his genera 
Chelyophorus and Actinolepis, to which you refer, J] cannot, in the 
absence of materials on which to form a judgment, venture to come 
to any conclusion. I have not much confidence, however, in the 
specific characters founded on peculiarities of tubercle, unless very 
striking indeed ; as I have seen the forms of these vary in the plates 
of even the same individual, whether Coccosteus, Pterichthys, or As- 
terolepis. In conclusion, I must be permitted to say, that my re- 
pugnance to traverse in aught the findmgs of the distinguished 
naturalist of Neuchatel would be really insurmountable, were I not 
convinced that he himself would, of all men, be the first to assert the 
necessity of leaving open an appeal from Agassiz to Nature.” 
The foregomg description, so remarkable for anatomical detail, 
mechanical argument, and graphic illustration, requires no comment 
from me. It gives great weight to the observations I had previously 
recorded in every instance where its author coincides with them ; 
and in the singular case in which he differs from, or rather, I should 
say, exceeds them, namely in the detailed description of the relations 
between the dorsal plate and the surrounding plates, the mechanical ar- 
gument I founded on my limited evidence is immeasurably strengthened 
by his more extended investigation. The ball and socket joint to 
which he alludes, by which the pectoral fin was articulated, was, 
strictly speaking, only so anteriorly ; the posterior limb of the ball 
was hollowed out for the reception of a bony process extending from 
the socket, somewhat similar to the odontoid process of the human 
axis (see fig. 2, 2), giving great strength to the jomt, and providing 
against dislocation without materially constraining the power of motion 
in those directions in which the organ would require to be exercised. 
In a subsequent communication Mr. Miller says, “I have suc- 
ceeded in tracing to its origin the Pamphractus of Agassiz. The 
specimens which he figures (Old Red, tab. 4. figs. 4, 5, 6) could never 
have furnished the materials of his restoration. These materials he 
evidently derived from the print of a Pterichthys of the Upper Old 
Red (showing the dorsal superficies of the creature), given by the 
Rev. Dr. Anderson of Newburgh, in his Essay on the Geology of 
Fifeshire (Quarterly Journal of Agriculture, vol. xi. 1840), as that of 
a fossil beetle. You will see, on comparison, how true it is, in the 
position and general form of the plates, (with the exception of the divi- 
sion of the dorsal plate at what is simply a strongly-marked groove,) 
to the upper surface of Pterichthys. It is not equally true, however, 
2a2 
