324 Canadian Record of Science. 



pidian fishes, however, makes it probable that this is not 

 the correct view to take of the position of these plates rela- 

 tive to the dorsal and ventral sides of the creature which 

 they covered. 



In JPteraspis (Kner) which, however, is a Devonian and 

 not a Silurian genus, there is a considerable resemblance in 

 the ornamentation of the test, and generally in the dorsal 

 armature, to the covering plates of the Acadian fish ; but if 

 we attempt to compare the different plates of which the 

 shield of our Pteraspidian fish is composed, with those of 

 Pteraspis, they will be found to differ widely from that 

 genus. On the other hand, if the plates a, b and c be com- 

 pared with those of the genus Ci/athaspis (Lank.), the cor- 

 respondence of parts is striking. 



In Prof. E. Pay Lankester's monograph on the fishes of 

 the Old Bed Sandstone,* we appear to have only one ex- 

 ample of this genus described, for although Cyathaspis (?) 

 Symondsi is described under this head, it seems very doubt- 

 ful whether it should be so referred. The typical species 

 0. Banksii (H. & S.) is Silurian, and possesses a set of plates 

 quite analogous to those of the Acadian fish. There is also 

 on the central plate (" dorsal scute ") a tubercle, indicated 

 in Prof Lankester's figures, which holds the place of a 

 similar circular elevation on the shield of the Acadian fossil 

 (see fig. c). In Prof. Lankester's examples of C. Banksii, 

 the surface markings appear to have been obscure, except 

 on the rostral and lateral plates ; we do not know, there- 

 fore, how far the markings on tbe main plate of each of 

 these two fishes were similar. In the Acadian species the 

 triangular space between the tubercle above referred to and 

 the front of the chief hexagonal plate c possessed a group 

 of sinuous and looping, mostly transverse, strise, differing 

 in direction from the longitudinal stria? that mark the rest 

 of the plate. 



Curiously enough, the plate d possesses characteristics 

 analogous to those of the scute of the genus Scaphaspis 

 (Lank.). The markings on the surface of this plate are 

 almost exactly parallel to those of the "dorsal" scute of 



* Memoirs of the Palteontographical Society, London, vol. xxi. 



