On the Structure of Coccosteus decipiens, Afjassiz. 211 



XXII. On the Structure 0/ Coccosteus decipions, Af/assiz. 

 By Dr XL H. Tkaquaik, F.ll.S, F.G.S. [Plate XL] 



(Read 18th December 1889.) 



In a paper (13) on Homostetcs published in the Proceedings 

 of this Society for Session 1888-89, I entered into the 

 structure of Coccosteus so far as was necessary for the purpose 

 of instituting a comparison between the two genera. In the 

 present communication I propose to consider the structure of 

 Coccosteus in greater detail. 



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

 is reproduced in PL XL, 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 coufounded with them. Quite recently, however, 

 the Edinburgh Museum has acquired a small collection of 

 (7occos^e?/.s-remains from Stromness, in Orkney, in which the 

 details of the surface of the cranial plates are most beauti- 

 fully shown, and are entirely corroborative of the sketch 

 which I published a year ago. 



As I have previously stated (12, p. 511), I retain only two 

 species of Coccosteus from the Scottish Lower Old Eed Sand- 

 stone, namely C decipiens, Ag., and C minor, H. Miller, the 

 differences which have led to the separation of " oUongus^' 

 Ag., ^' cusjnda.tics," Ag., microspondylus, trigonaspis, and 

 pusillus, M'Coy, and Milleri, Egert., being dependent either 

 upon the mode of preservation or upon trivial variations in 

 the shape of certain plates, which are extremely common up 

 to certain limits. That which I find especially difficult to 

 understand is how Prof, von Koenen (10) should propose to 



