CEPHALASPIDiE. 13 



region, the eyes being placed centrally. In those genera of Heterostraci which 

 develop a more complex shield the evolution shows itself in the enlargement of the 

 anterior region, into a rostrum, orbital pieces, &c. In those Osteostraci which exhibit 

 a shield of more than one piece the complexity results from the diminution of the orbital 

 region and cornua, and the development of a large posterior plate, which is represented 

 by only a small portion of the simple shield. 



§ V. Zoological Position. — Whilst no one has ever doubted that Cephalaspis Lyellii and 

 its allied forms are Pishes, in the face of the specimens showing the body and fins, the Hete- 

 rostraci ha\e been sometimes regarded as Cephalopods and Crustacea as related above. After 

 his examination of the structure of Fleraspis, Professor Huxley remarks^ — " No one can, I 

 think, hesitate in placing Pteraspis among Fishes. So far from its structure having ' no 

 parallel among Pishes,' it has absolutely no parallel in any other division of the Animal 

 Kingdom. I have never seen any Molluscan or Crustacean structure with which it could 

 be for a moment confounded." He is then led to conclude that Pteraspis is a Pish from 

 its close relation in structure to Cephalaspis. Above, the differences between the two have 

 been dwelt on ; the resemblances have to be noticed as forming the original reason for 

 retaining Pteraspis among Pishes. In each the shield is excessively thin, and composed of 

 three or four layers: 1st, an 'internal,' composed of lamellae parallel with the surface, 

 and traversed more or less obliquely by vascular canals; 2nd, next to this a 'middle 

 layer' containing the network of wide canals, or polygonal sinuses, the upper floor of which 

 forms a 'reticular layer' in Pteraspis; 3rd, the 'external layer,' consisting of a cosmine, 

 like substance raised into ridges or tubercles. Later conclusions as to the form of 

 the shield in some Heterostraci, and the discovery of the scales of one species, have 

 rendered the piscine nature of Pteraspis undeniable. 



With regard to the relations of the family Cephalaspida to other groups of Pishes, I 

 cannot do better than quote the opinion of Professor Huxley, the only one current at the 

 present time on the subject. It nmst be borne in mind in connection with this question, 

 that no indication whatever of an internal skeleton belonging to these Pishes has been 

 found ; and that specimens showing the body of Cephalaspis prove by negative evidence 

 that such a structure did not exist. While assigning a distinct family to the genera 

 Cephalaspis, Pteraspis, Auchenaspis, and Menaspis, he remarks ■: " No one can overlook 

 the curious points of resemblance between the Siluroids, Callichthys and Loricaria, on the 

 one hand, and Cephalaspis on the other, while in other respects they may be still better 

 understood by the help of the Chondrostean Ganoids. Compare, for example, Scapi- 

 rhymlms with Cejjhalaspis, or the great snout of Pteraspis with that of Spatularia. I am 

 inclined to place the Cephalaspida provisionally among the Chondrostei, where they will 

 form a very distinct family." 



1 Op. cit., p. 277. 



2 ' Memoirs of the Geol. Survey of Great Britain, Decade X,' p. 3S. 



