116 IOWA GEOLOGICAL SURVEY 



termination of the costae suggests that it was exposed for a 

 considerable distance. The distal third of the posterior face is 

 armed with a double series of stout, decurved hook-like denticles, 

 of which 16 may be counted in the actual condition of the speci- 

 men. The extreme apex of the spine is wanting, and the distal 

 portion for the space of about 2 cm is preserved in impression. 



This elegantly formed and beautifully enamelled ichthyodo- 

 rulite has been only recently brought to light by Dr. J. M. Clarke, 

 the accomplished New York State geologist, to whom we are 

 indebted for the privilege of studying and describing it. Found 

 in the Stafford limestone at Stafford, New York, a rock which 

 lies within the black Marcellus shales and carries in fine develop- 

 ment the first clear representation of the Hamilton fauna which 

 develops more profusely above, the present example not only 

 acquaints us with a new species of Acanthodian sharks, but also 

 one that is eminently proemial, according to Dr. Clarke's defini- 

 tion of the term. That is to say, the species which is here found 

 at the base of the Erian marks the initial occurrence of a group 

 which reappears sparsely from time to time at different intervals 

 in the Devonian, but does not acquire ascendency in the western 

 hemisphere until during the Carboniferous. It should be said, 

 however, that this statement would need qualification in case the 

 suspected Acanthodian affinities of Machaeracanthus should be 

 definitely established by future discoveries. Account should 

 also be made of the fact that the Lower Devonian fish-bearing 

 horizon at Campbellton, New Brunswick, has not been precisely 

 correlated with the Appalachian formational series, nor has in- 

 termigration of the vertebrate elements been shown to take place 

 between the two provinces. 



At this point, however, it is pertinent to recall that several 

 undoubted Acanthodian species, including one of Gyracanthus, 

 have been described from the Campbellton locality in New Bruns- 

 wick, and that, judging from Traquair's rather meagre descrip- 

 tion, the spine called by him G. incurvus presents some similarity 

 to the new form from the Marcellus made known above. Dr. 

 Traquair's specimen barely exceeds 5 cm (2% inches) in length, 

 and its distinguishing feature is stated to consist in "an antero- 

 posterior curvature of a very much stronger and more pro- 



