14 Mr. A. S. Woodward on the 



Family Onychodontidse. 

 Genus Onychodus. 



Onychodus arcticus, A. S.Woodward. (PI, IT. fi^. 12.) 



1880. Onychochts arctiats, A. S. Woodward, Rep. Brit. Assoc, p. 535, 

 aud Ge'ol. Mag. [8J vol. vi, p. 409. 



The presyraphysial bone thus described still remains unique, 

 but an opportunity is now afforded for publishing the drawing 

 of the specimen given in PI. TI. fig. 12. Tiiis figure is of 

 twice the natural size, and exhibits the characters already noted 

 in the original description. 



Form, and Loc. Ironstone, Mimers Valley. 



IncertcB sedis. 



In addition to the dermal plJltes of Psammosteus and the 

 bones of Crossopterygian Ganoids the Ironstone of Mimers 

 Valley also furnishes numerous large and robust plates, which 

 appear as yet to be incapable of determination. A few of 

 these are marked with coarse closely arranged tuberculations, 

 which occasionally pass into ridges (Lankester, op. cit. pi. iv. 

 fig. Ifi) ; and their tissue, though not well preserved, seems 

 to have been dense. The majority of the plates, however, 

 are of a different character, exhibiting a relatively thick 

 middle layer of polygonal cancellaj, which is traversed by 

 straight closed canals, sometimes few, sometimes numerous, 

 and now filled with mineral matter. The outer and iimer 

 surfaces of these ])lates, so far as can be observed, are smooth, 

 and the borders always become attenuated, as if ailjoining 

 elements were originally united by overlap. ^lost of t!ic 

 plates are nearly flat, only upturned occasionally at some of 

 the borders ; but one specimen is very strongly bent and 

 keeled and thickened along the ridge. So!ne of the elements 

 were distinctly arranged in symmetrical pairs ; and one torin 

 of plate is especially suggestive of the ventro-lateral of an 

 Asterolepid fish. 



Microscopical sections of these plates exhibit no bonc- 

 lacuna3 in the tissue of the middle layer ; and it has not been 

 possible to make a satisfactory examination of the external 

 layers. However, the extremely vascular character of the 

 tissue seems to justify the reference of these fossils to an 

 unknown large Ostracoderm ; and the writer is inclined to 

 suspect that they may eventually prove to represent an ally 

 of the genus Cerai^pis, wliicii occurs in the Devonian of the 



