520 OBSERVATIONS ON THE GENUS PTERASPIS 



the anterior part of the dorsal region of the body. Apparently 

 anchylosed or continuous with the anterior edges of some specimens 

 of these plates, there is a bony disk, prolonged at its postero-lateral 

 angles into two long cornua, which pass into the edges of the oblong 

 plate. The disk exhibits the characteristic structure and the peculiar 

 striated sculpture ; it is prolonged anteriorly into a sort of rostrum, 

 whose length varies with the species of Pieraspis : laterally it exhibits 

 two well-marked nearly circular marginal apertures, which I make no 

 doubt are the orbits. 



I conceive, therefore, that the part in question corresponds with the 

 anterior part of the cephalic disk of Cephalaspis, and that the oblong 

 plate, which may well be termed the occipito-nuchal plate, answers 

 partly to the posterior moiety of the cephalic disk of Cephalaspis, 

 partly to that median backward prolongation of the posterior margin 

 of the cephalic disk of Ceplialaspis to which I have particularly directed 

 attention in my memoir. It is from this that the strong nuchal spine 

 of the Cephalaspis arises ; and as if to render the resemblance com- 

 plete, the most perfect specimens of Pteraspis show that it had a like 

 spine developed in a similar position. 



There exists, indeed, a most interesting gradational series of forms 

 between Ceplialaspis Lyellii and Pteraspis. 



The Aiichenaspis of Sir Philip Egerton has a cephalic disk with 

 the semicircular anterior outline of that of Ceph. Lyellii, but the inter- 

 space between the cornua of the disk is nearly filled up by the large 

 nuchal plate, whose backward extent may have been greater than is 

 exhibited by any of the specimens of Auchenaspis yet obtained. 

 Enlarge the nuchal plate of Auchenaspis and give the cephalic disk 

 -a more produced anterior outline, and the result is the form of 

 Pteraspis. 



Throughout all these gradations of form, however, it must be 

 borne in mind that the minute structure of the parts is such as 

 at once to enable the observer to distinguish Cephalaspis from 

 Pteraspis. 



I ventured in my paper, read before the Geological Society, to 

 -say that the Ganoid nature of Cephalaspis and Pteraspis appeared to 

 me to be unproved, and to allude to the relations between these genera 

 and the great group of existing Siluroidei. Without at all wishing to 

 push too far resemblances which, when we come to know more, may 

 turn out to be mere analogies, I must say that the new facts which I 

 have brought for\\ard appear to point somewhat in the same direc- 

 tion. The siluroid fishes, in fact, are especially characterized by 



