OF SILUEIAN AND DBYONIAN. 53 



Lankester recognizes the analogy between the rhombic tubercle in Cyathaspis and the 

 small circular plate in Pteraspis, but offers no coujectiire as to the purpose of the latter. 



When we compare our species with those of the genus Scaphaspis of Lankester, points 

 of resemblance as close as those with Cyathaspis may be observed, but the resemblance 

 is to the features of the ventral shield only, of the Acadian species. The characters of 

 Scaphaspis are well represented by S. tnncatiis of the Dowuton Sandstone, which has the 

 same two-ranked ridges on the central part of its scute, as Diplaspis, while, as in it, the 

 borders are ornamented by ridges that are more equal in size ; the ridges of the centre of 

 the sciTte are also looped at the anterior end, as in those of the ventral scute of Diplaspis, 

 leaving a small anterior area where the ridges are transverse to its longer axis, and in 

 front of which the scute itself is truncated. ■ There appears also to be a want of sym- 

 metry in the two sides of this scute at the anterior end, as in the similar part of the 

 ventral scute of Diplaspis. 



Prof. Lankester relies upon the peculiar two-ranked arrangement of the ridges of the 

 scutes of Scaphaspis and Cyathaspis to distinguish those that belong to the Silurian age 

 from those that are Devonian. Not only is there such a i^eculiarity, biit the ornamenta- 

 tion of the scutes in the Devonian species is carried out upon a different plan from those 

 of the Silurian. 



From the preceding description and the figures given herewith, it will be seen 

 wherein the peculiar ornamentation of the Silurian species consists. In the Devonian 

 species of Scaphaspis the ornamentation is concentric to the elevated ridge on the poste- 

 rior half of the scute, and the ridges run around the front of the scute in a regular 

 manner, and are of about equal size. In the Devonian species there is also a set of ribs 

 or moderately raised portions of the test, diverging from the front of the elevated ridge 

 above referred to, to the anterior of the scute, and this part of the scu^te is more elongated 

 than in the Silurian species. So radical are the differences between the ornamentation 

 of the scutes of the Devonian species of Scaphaspis and those of the Silurian, that the 

 two must belong to different genera. 



But in the points in which the ornamentation of the scutes of the Devonian species 

 of Scaphaspis differs from that of the Silurian, it agrees with the ornamentation of 

 Pteraspis, and this genus also has the radiating ridges on the anterior end of the dorsal 

 scute which we observe in the Devonian species of Scaphaspis. In both genera also the 

 scutes grew by regular additions to the sides and anterior ends of the scutes. 



In their form, therefore, their mode of growth and their ornamentation, the Devonian 

 scaphaspid plates have the same relation to the genus Pteraspis that the Silurian sca- 

 phaspid plates had to the genus Cyathaspis, and we cannot but suspect that the scutes on 

 which the genus Scaphas^iis is founded are simply the ventral sciites of species of other 

 genera. 



In this view the analogy of the parts in Diplasx^is to the Silurian genus Cyathaspis 

 is clear and pronounced, though we find it to differ in the following respects : the lateral 

 cornua or plates are in two pieces in place of one, and the ocular plates of Cyathaspis are 

 apparently wanting in Diplaspis. 



In comparing Diplaspis with the fish-plates (Palseaspis) found by Prof Claypole in 

 the variegated (Onondaga) Shale of Pennsylvania,' while there is a general resemblance 



' Quarterly Journal Geological Society, London, Feb. 18S5. 



