﻿INTRODUCTION. XV11 



problematical ; and it has thus been deemed convenient to treat 

 next in order the great extinct group of Chordate animals to which 

 Prof. Cope l has applied the name of Ostracodermi. These pertain 

 either to the Class Pisces or to some lower denomination yet to be 

 determined. 



Though placed in immediate association with the TJrochorda and 

 Agnatha by Cope, and lately supposed to be allies of the Arachnids 

 by Patten 2 , few facts can be advanced in favour of either of these 

 theoretical interpretations of the group. The Arachnid theory is 

 based upon a complete misapprehension of the most fundamental 

 points in Ostracoderm skeletal anatomy 3 ; while the comparison of 

 the dorsal opening in the cranial shield of the Asterolepidae with 

 the mouth of an Ascidian, as originally made by Cope 4 , is already 

 admitted by that author himself 5 to prove untenable. That there 

 were no hard parts round the mouth and in relation to paired 

 appendages capable of being preserved under the ordinary condi- 

 tions of fossilization seems to be satisfactorily demonstrated ; but 

 there is no justification for any further statement that jaws, pectoral 

 and pelvic arches were absent. On the other hand, a symmetrical 

 paired series of lateral indentations on the visceral aspect of certain 

 Ostracoderm dorsal shields (e. g., Cyatliaspis 6 ) suggests the original 

 presence of well-separated gill-pouches, between which it is reason- 

 able to infer there were supporting elements of the nature of visceral 

 arches. There is a distinctly movable flap or plate at the posterior 

 opening of what appears to have been a common gill-cavity outside 

 these pouches in some genera (e. g., Cephalaspis and Ptericlitliys). 

 In every instance when the plate between the orbital apertures can 

 be distinctly observed there is a small deep pit on its visceral 

 aspect, sometimes projecting as a tubercle externally ; and this 

 occupies the precise position that would have been held by the 

 pineal body of a vertebrate brain, had such been present. A pair 

 of > -shaped impressions on the visceral aspect of the dorsal shield 



1 E. D. Cope, Amer. Nat. vol. xxiii. (1889), p. 852. 



2 W. Patten, Quart. Journ. Micro. Science, vol. xxxi. (1890), pp. 359-365, 

 fig. 13. 



3 A. S. Woodward, Ann. Mag. Nat, Hist. [6] vol. vi. (1890), pp. 314-316. 



4 E. D. Cope, Amer. Nat. vol. xix. (1885), p. 290. 



5 E. D. Cope, ibid. vol. xxii. (1888), p. 915. 



6 See especially the figures by Kunth, Zeitschr. deutsch. geol. Ges. vol. xxiv. 

 (1872), pi. i . fig. 1 ; A. von Alth, Abh. k. k. geol. Reichsanst. vol. vii. no. 1 

 (1874), pi. v. fig. 1 ; and Lankester, ' Cephalaspidee ' (1868), pi. ii. fig. 11. 



PAKT II. b 



