316 
On a New Theory of Pterichthys. 
and the metasternum of Scorpions and Merostomata. Again 
we fail to find any justification for this statement in Dr. 
Traquair’s restoration, the accuracy of which we have been 
able to verify in every respect. Moreover, when the conclu- 
sion is reached that u since their eyes are situated on the 
haemal surface . . . Pterichthys , Bothriolepis , &c. are nearer 
related to the Arachnids than to the Vertebrates,” it is obvious 
that some of the most fundamental characters in the skeleton 
of the first-named genus have been overlooked. 
Even in the dorsal shield itself there are features inexplic- 
able except on the supposition that it covered the neural 
aspect of an organism provided with a typical vertebrate 
brain. The plate between the eyes, for example, exhibits a 
deep pit on its visceral surface identical in position with that 
which few will deny received the pineal body in several early 
shielded types (e. g. Coccosteus ), which are proved to be 
vertebrates by the discovery of the axial skeleton of their 
trunk. But the characters of the tail of Pterichthys , now 
well known, seem to the present writer absolutely conclusive 
of the relations of the neural and hsemal aspects. As shown 
by Dr. Traquair, this tail is fish-like in every respect ; it has 
ridge-scales and a median fin on the border that continues 
the convexity of the eye-bearing shield, and the pointed 
extremity of the tail is turned upwards towards this border. 
Moreover, at least one specimen in the British Museum proves 
that there was a large terminal fin extending chiefly on the 
convex border of the extremity. Such structures are unparal- 
leled in any known group except that of the fishes ; and when 
they do occur here the produced body-lobe of the heterocercal 
tail is invariably directed towards the neural aspect, while 
the ridge-scales and median fin, when present only on one 
border, are without exception on the same aspect. 
A tail of a closely similar character is also known in the 
allied family of Cephalaspididse, and it seems to the present 
writer proved beyond doubt that all the organisms of this 
type are true Chordata, while many probably reach the phase 
to which the term Vertebrate is now commonly restricted. 
Indeed, as nearly all the special points noticed by Mr. Patten 
result from a consideration of insufficient or inaccurate data, 
it seems needless to follow him further in his wide generali- 
zation as to the arrangement of the exoskeleton and eyes in 
the lower vertebrates. There is much parallelism in the 
skeleton of totally distinct groups that yet remains to be 
explained ; and it seems quite as philosophical to us to infer, 
from the known anatomy of a cockle, that the valves in the 
extinct Spirifer were lateral shields, as to interpret mere 
superficial resemblances in the armour of Pterichthys and 
Eurypterids as homologies. 
