THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[SIXTH SERIES.] 

 No. 26. FEBRUARY 1890. 



XVI. — On the Structure o/Coccosteus decipiens, Agassiz. 

 By R. H. Tkaquair, M.D., F.R.S.* 



[Plate X.] 



In a paper on Homosteus (13) published in the 'Geological 

 Magazine ' for January 1889 I entered into the structure of 

 Coccosteus so far as was necessary for the purpose of insti- 

 tuting a comparison between the two genera. In the present 

 communication I propose to consider the structure of Cocco- 

 steus in greater detail. 



The figure which I gave in that paper of the cranial shield 

 is reproduced in PI. X. fig. 2, with the addition of the dorsal 

 cuirass. It is, I believe, accurate, and represents the result 

 of a close study of a very great number of heads. Compara- 

 tively few specimens are, however, available for the purpose, 

 those especially from Lethen and most of those from Orkney 

 being ill-adapted for following the sutures separating the 

 plates, while Cromarty and Edderton furnish those in which 

 the surface is most perfectly preserved, thus affording the best 

 opportunity for accurately distinguishing the true sutures from 

 those superficial grooves which in past times have been so 

 often confounded with them. Quite recently, however, the 



* Read before the Royal Physical Society of Edinburgh. 18th Decem- 

 ber, 1889. 



Ann. & Mag. N, Hist. Ser. 6. Vol. v. 10 



