14 Mr. A. S. Woodward on the 



Family Onychodontidse. 

 Genus Onychodus. 



OnijcJiodm arcticus, A. S.Woodward. (PI. TI. fig". 12.) 



1880. Onychodus arcticus, A. S. Woodward, Hep. ]>rit. As.^oc. p. 585, 

 aud Ge'ol. Mdg. [8 J vol. vi. p. 490. 



The presymj)liysial bone thus described still remains unique, 

 but an opportunity is now afforded for publishing- the drawing- 

 of the specimen given in PI. II. fig. 12. This figure is of 

 twice the natural size, and exhibits the characters already noted 

 in the original description. 



Form, and Loc, Ironstone, Mimers Valley. 



Tncertce sedis. 



In addition to the dermal plates of Psammosteus ami 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. 16) ; 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 cancella^, wliicli is traversed by 

 straight closed canals, sometimes few, sometimes numerous, 

 and now filled with mineral matter. The outer and iimer 

 surfaces of these plates, so far as can be observed, are smooth, 

 and the borders always become attenuated, as if adjoining- 

 elements were originally united by overlap. Most of the 

 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. Some of the elements 

 were distinctly arranged in symmetrical pairs ; and one form 

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

 Asterolepid fish. 



Microscopical sections of these plates exhibit no bone- 

 lacunfe 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 tiiese 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 Ccrasjn's, which occurs in the Devonian of the 



