1S98-|902. N(x3a]^ U PPER D EVONIAN FISH REMAINS. 37 



skeleton elements of Psammosteus cannot be determined with certainty, 

 when we attempt to localise them according to the well-known dermal 

 skeleton of Drepaiiaspis. 



Fragments of plates with one side covered with denticles, may of 

 course be regarded as portions of the large body plates. In the same 

 way the small plates wilh a large mid-denticle which I have described 

 may also with great probability be considered as corresponding to the 

 small intermediate plates of Drepanaspis. The characteristic skeleton 

 that Pander depicts in his PI. 7, fig. 16, may be regarded as a dorsal 

 spine, and others may be considered to be fulcra. The remarkable 

 specimen described and depicted by Aga.ssiz as Ps. parodoxiis. may be 

 assumed to be scales from the caudal region. 



On the other hand, it is more difficult to deteimine compressed and 

 asymetric spine shaped elements of which I have described a fragment 

 under Ps. arcticus (PI. V, figs 4—5). IF compared with Drepanaspis. 

 it is scarcely possible to regard this either as a dorsal spine or as a 

 fulcrum. I believe that it is more probably a portion of a corner spine, 

 cornua in this form having possibly been extended to comparatively 

 short or long spines ^. The difference that here becomes apparent 

 between Drepanaspis and Psammosteus, was presumably also present 

 in other characters. The family Drepanaspis must therefore be 

 retained. 



In consideration of the circumstance that Drepanaspis Gemiinde- 

 nensis, Schlut. and Psammosteus arcticus Kijeh must be regarded as 

 belonging to different genera although the structure of the dermal skeleton in 

 both cases is almost identical, we have good grounds for assuming that 

 Psammosteus complicatus Kler, wilh even greater probability forms 

 a separate genus on account of the dermal skeleton. The intermediary 

 bundles of dentine tubules, which are seen in the skeleton plates in the 

 latter form, are not known in any other. It seems to me to be probable 

 that this character has been accompanied by other diverging characters. 

 For the present however, I do not propose to make a new genus: 

 judging from other circumstances this must at all events have been 

 closely i-elated to the others. 



From our present knowledge we must therefore assume that all 

 these forms constitute a natural group, which may best be denominated 

 the Psammosteidae family. In this we find, of course, several lines of 

 development, which have evolved independently of each other from 



1 Vide also Traquaie's Report on Fossil Fislies in Silurian Rocks of Hie South of 

 the Sonth of Scotland. Trans. Roy. Soc. Edin. XXXIG, 1898. 



