228 ProcecdiiKjs of the Ruyal rJiydcal Sucietij. 



niavked differences between the two forms that it is thought 

 more prudent, for the present, to distinguish the Canadian 

 species by a local name." He notices the similarity in the 

 arrangement of its " superficial " {i.e., sensory) grooves with 

 those of C. decipiens, Ag., and finishes by saying : " It would 

 seem, therefore, that C. Acadicus may be distinguished from 

 O. decipiens by the different shape of the post-dorso-median 

 plate, from C. cuspidatus by the different arrangement of the 

 grooves on the outer surface of its cranial shield, and from 

 both by the peculiar sculpture of its bony plates." ^ 



It is, therefore, evident that Mr Whiteaves was unaware 

 that G. cuspidatus is notliing but a mere synonym of 

 C. decipiens, and that he also makes no allowance for the fact 

 that Hugh Miller's figure on plate iii. of the " Old Eed 

 Sandstone," from which he seems to derive his information 

 as to the character of this supposed species, is only an im- 

 perfect restoration of C. decipiens executed at a time when 

 our information on such matters was still rather undeveloped. 



The cranial shield of Phlyctccnaspis Acadicus (Plate XII., 

 Figs. 1, 2) must have been considerably vaulted from side to 

 side, as the specimens, now much flattened, not unfrequently 

 present irregular longitudinal fractures. The form is broadly 

 ovate, truncated behind with prominent postero-lateral angles 

 (P.L.). In front of the postero-lateral angle the margin passes 

 obliquely outwards and forwards for a short distance, and 

 then forms anotb.er obtuse angle, the postero-external (P.E.), 

 succeeded by a shallow notch, in front of which is the antero- 

 external angle (A.E.). Immediately after this the direction 

 of the margin is forwards and slightly inwards to what may 

 be called the post-orbital angle (P.O.), whence proceeding 

 more strongly inwards, it forms a slightly excavated edge, 

 evidently equal to the orbital excavation of the shield of 

 Coccosteus, and bounded in front by the ante-orbital angle 

 (A.O.). Between the ante-orbital angles of opposite sides, 

 the margin of the shield is completed in front by a shallow 

 concavity occupied in the perfect state by the " rostral " 

 plate as shown by Mr WJiiteaves. 



Leaving the sensory groove system out of consideration for 



1 Trau.s. Roy. Soc. Can., vol. vi., sect, iv., 18S9, p. 93. 



