OE THE AMERICAN PTERASPIDIAN, PAL^ASPIS. 553 



The presence of the lateral plates brings Palceaspis into very 

 close relationship with Cyathaspis, and unless similar parts should 

 be found in Holaspis it will form a character of generic distinction 

 between these, in which case the last-named can no longer be 

 included in the American genus. It is, however, possible that 

 further search may prove that Holaspis did possess such parts, and 

 the question must therefore not be prejudged. Lankester's figure 

 and description certainly indicate no ' cornua/ and his explicit state- 

 ment that the fossil is intermediate between the more complex 

 Pteraspids and Scaphaspis sufficiently indicates his opinion on this 

 point. 



Of the remarkable canal-system of Holaspis so clearly shown in 

 Lankester's figure I have found no trace in Palceaspns, nor yet of the 

 ' pores ' accompanying the canals. 



Pig. 5. — Outline of ventral shield, from several specimens. 



front 



[Outer view, right side, showing obtuse angle.] 



In another point, however, the two fossils are in agreement. 

 There is in both a total absence of a separate rostral plate. Not 

 the slightest sign of division is shown in any of my specimens, and 

 in the representation of Holaspis accompanying Prof. Lankester's 

 paper the place of junction of such a plate with the dorsal shield is 

 equally unmarked. In both the striation flows on uninterruptedly 

 over the surface, and thus aff'ords a distinctive character from 

 Cyathaspis. Moreover the fore margin is in both well rounded and 

 complete, giving assurance that no such plate is missing. 



In no one of the many specimens that I have examined have I 

 seen the slightest indication of a medial posterior spine on the dorsal 

 shield, so that we may be quite confident that no such appendage 

 existed. There is moreover no thickening of the material, no con- 

 verging or change in the direction of the striae towards the hinder 

 end, and no sign of a furrow or groove for its insertion, such as 

 might have been looked for had Palceaspis possessed the spinous 

 process that characterizes Pteraspis and Cyathaspis. 



YIII. The Lateral Organs, ' Pins,' of Paljeaspis. 



In addition to the above-mentioned new characters of Palceaspis 

 there is another point on which I am at present unable to speak 

 with certainty, but the interest of it is too great to allow of its 

 omission. 



I have from time to time detected on the slabs containing the 



