126 ,-*- Dr. R. H. Traquair on the 



Edinburgh Museum has acquired a small collection of Cocco- 

 s^s-remains from Stromness, in Orkney, in which the details 

 of the surface of the cranial plates are most beautifully 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 Red Sand- 

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

 the differences which have led to the separation of " oblongus," 

 Ag., " cuspidatus" 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 

 remove C. Milleri, Egert., and C. pusillus, M'Coy, from 

 Coccosteus altogether, placing them in Brachydrirus, the fact 

 being that they are simply synonyms of decipiens, Ag. C. 

 minor, H. Miller, once mixed up with C. pusillus, M'Coy, may 

 possibly have to be put into a new genus on account of the 

 structure of the vertebral column, which presents an appear- 

 ance as if possessed of ossified centra * ; but I can see no 

 reason for associating this species with v. Koenen's Brachy- 

 deirus. 



The following description of the structure of the bony 

 skeleton of Coccosteus is therefore based upon an examination 

 of the common and well-known species C. decipiens, Agassiz. 

 Head. — In PL X. fig. 2 the bones forming the cranial 

 shield are sketched, as well as the ramifications of the lateral- 

 line grooves. These bones are : — one median occipital (m. o.), 

 two external occipitals (e. o.), two central plates (c), two 

 marginals (m.) } two postorhitals (pt.o.),t\vo preorbitals (p.o.), 

 one posterior ethmoidal (p. e.), and one anterior ethmoidal 

 (a. e.), between which last and the premaxillas (p.mw.) the 

 nasal openings (n.) are observable. I have already (13, p. 5) 

 explained that I have applied those names without the inten- 

 tion of considering any of the bones exact equivalents of 

 bones similarly named in ordinary fishes. 



The orbit, the upper margin of which is formed by the exca- 

 vated outer edges of the post- and preorbital buckler-plates, is 

 completed below by the superior maxillary bone (m.x. fig. 1), 

 which strongly resembles in shape that of typical Palseonis- 

 cidee in being broadly expanded behind, where it covers the 

 cheek, and suddenly excavated to form a tapering process 



* This is apparently the species " with a true bony vertebra " referred 

 to by Murchison in ' Siluria, 3rd ed. p. 504. 



