Nexo Theory o/* Pterichthys. 315 



Mr. Patten, tliat naturalists must seek the ancestors of the 

 Chordati' pliyhun. 



However plausible the theory and however convincing tlic 

 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. AJore especially docs it seem clear that the 

 dermal plates in the fossils just mentioned cannot be inter- 

 j)reted 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 luemal 

 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 lia^mal shield of a Trilobite and compares it with 

 corresponding outlines of the dorsal (*' hasmal ") shield of 

 Pterichthys and Bothriolejns. 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. IJ. 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 Bothriolejns exhibits a 

 cervical suture, proving the concrescence of vagus segments ; 

 but the groove in qnestion 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 j 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 " 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- 

 lepidfe," Ann. & Mag. Nat. Hist. [6] vol. ii. (1888), pp. 485-604, 

 pis. xvii., xviii. 



