New Theory 0 /* Pterichthys. 315 
Mr. Patten, that naturalists must seek the ancestors of the 
Chordate phylum. 
However plausible the theory and however convincing the 
arguments deduced from the morphology and embryology of 
existing types, we venture to think that Pterichthys and 
Bothriolepis cannot be cited as having any distinct bearing on 
the subject. More especially does it seem clear that the 
dermal plates in the fossils just mentioned cannot be inter- 
preted as the homologues of certain plates of the Arachnids, 
in the manner the author supposes ; and when it is suggested 
that the so-called dorsal shield of Pterichthys is on the haemal 
aspect of the animal, an ichthyologist, at any rate, is unable 
to regard the statement as anything beyond unjustifiable 
speculation. 
In the first place, Mr. Patten gives outline-sketches of the 
anterior haemal shield of a Trilobite and compares it with 
corresponding outlines of the dorsal (“ haemal ”) shield of 
Pterichthys and Bothriolepis . Unfortunately, however, the 
latter are copied from old erroneous figures, the inaccuracy of 
which was pointed out some time ago in these pages by Dr. 
R. H. Traquair *. The agreement in general size and shape 
is first insisted upon ; but that, it must be admitted, is a 
circumstance of very secondary importance. In the second 
place it is stated that, like that of the Trilobite, the “ cepha- 
lothoracic ” shield of Pterichthys and Bothriolepis exhibits a 
cervical suture, proving the concrescence of vagus segments ; 
but the groove in question is shown by overwhelming evi- 
dence to be nothing beyond a superficial slime-canal, evidently 
connected with the sensory system. The same remark 
applies to the inner of the “ great semicircular sutures 
extending parallel with the edge of the shield around the 
front and sides,” which is another point of supposed similarity 
insisted upon ; the so-called outer semicircular suture repre- 
sented in Pterichthys (evidently after Pander) does not exist. 
The '‘ocular plates” and “facial suture” certainly are in 
part comparable ; and there is some fanciful resemblance of 
the median plates to the median lobes of a Trilobite, but the 
comparison does not appear very satisfactory. 
Having thus disposed of what is assumed to be the hsemal 
shield, Mr. Patten remarks that the u neural surface of 
Pterichthys , or the neural surface of a true fish/’ has “ the 
median cranial plates arranged in pairs, terminating in a 
posterior unpaired plate,” corresponding to the coxal plates 
* R. H. Traquair, “ On the Structure and Classification of the Astero- 
lepidse,” Ann. & Mag. Nat. Hist. [6] vol. ii. (1888), pp. 485-504, 
