56 Dr. R. H. Traquair — A Neio Genus of Coccosteidce. 



It is, therefore, evident that Mr. Whiteaves was unaware that C. 

 cuspidatus is nothing 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 Ked Sandstone," from which he seems to 

 derive his information as to the characters of this supposed species, 

 is only an imperfect restoration of C. decipiens executed at a 

 time when our information in such matters was still rather un- 

 developed. 



The cranial shield of Phlyctanms Acadicus (PI. III. Fig. 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 poster o-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 another obtuse angle, 

 the poster -o-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 maybe 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. Whiteaves. 



Leaving the sensory groove system out of consideration for the 

 present, it is first to be noticed that the determination of the con- 

 stituent plates of the buckler is a matter of extreme difficulty, from 

 the fact that they are apparently all fused or anchylosed together in 

 the manner in which those of the shield of Pteraspis are in the 

 adult form supposed to be. Mr. Whiteaves has noticed the frequent 

 arrangement of the tubercles in concentric rows, and this arrange- 

 ment, much more marked in some shields than in others, along 

 with the lines seen to radiate from the ossific centres in abraded 

 specimens, first led me to suspect that the form and arrangement 

 of the cranial plates differed in some material points from what is 

 given in Mr. Whiteaves' sketch. 1 Close observation by means of 

 a good lens enables one, however, also to observe the original lines 

 of suture, due care being taken not to be deceived by fractures. 

 Though the direction of these sutures is often indicated by depressed 

 lines or slight grooves free from the tubercular ornament, yet the 

 actual suture is not incised, but slightly raised like a very fine 

 thread, this being due to the manner in which the original lines of 

 separation between the bones have become entirely filled up by 

 osseous matter. 



The results obtained by noting these lines in connection with the 



concentric arrangement of the rows of tubercles being in all essential 



respects the same in all the specimens (six) which I have examined, 



and being furthermore in complete accordance with the information 



1 Op. cit. p. 93, woodcut. 



