316 On a New Theory q/Tterichtliys. 



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 " since their eyes are situated on the 

 hsemal surface . . . PterichthySj 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 Pterwhthys, now 

 well known, seem to the present writer absolutely conclusive 

 of the relations of the neural and hjemal 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 Cephalaspididte, 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 Sj>irifer were lateral shields, as to interpret mere 

 superficial resemblances in the armour of Pterichthys and 

 Eurypterids as homologies. 



