VERTEBRATES, 199 



from a locality consists not of a single form merely, but of indi- 

 viduals pertaining to the. same side in relation to their position upon 

 the jaws. However, fortunately, now and then appears a specimen 

 which seems to set at rest whatever doubts may have been enter- 

 tained as to the actual state of things, and in a few of the species 

 the collections are rich in data going to establish a more precise 

 and comprehensive understanding of the species themselves and the 

 family and ordinal relations of the genus. 



So far as has come to the notice of the writers, the genus Psam- 

 modus has been classed by authors with the Cestracionts ; Dr. L. de 

 Koninck, however, recognizing their distinct family character, for 

 which he proposed the term Psammodontidne.* As early as 1843, 

 Captain Jones, in a communication in reference to his acquisitions 

 of fish remains from the mountain limestone localities in the vicinity 

 of Armagh, says : "There are good specimens of one more tooth, 

 somewhat allied to Psammodus, yet having a distinct character, which 

 will connect it with the Myliobates;"! and to whom should be cred- 

 ited thus early the recognition of the affinities of the Psammodonts. 



The examination of a considerable suite of the teeth of Psammo- 

 dus must convince one of their having originally occupied a serial 

 position upon the jaws, stretching across from side to side, in the 

 manner exemplified by the Myliobates of Tertiary and existing seas. 

 Indeed, Dr. de Koninck alludes, loc. c., to a series of three teeth of 

 the species P. porosus, found by Mr. Neilson in the mountain lime- 

 stone deposits near Glasgow (N. B.), which still retain their relative 

 position to one another, and with which the distinguished palaeon- 

 tologist reconstructs the dental armament of the jaw. But perhaps 

 the most complete and satisfactory information to be derived from 

 isolated detached teeth is that secured by Mr. Springer of the species 

 bearing his name, from the Upper Burlington limestone of our Lower 

 Carboniferous series. The latter are so well preserved as to furnish 

 many and highly desirable details without which still some uncer- 

 tainty might intervene to a satisfactory conclusion on the chief points 

 relating to the disposition of the teeth upon the jaws. These show 

 conclusively that the teeth were arranged in at least double rows, 

 not exactly opposite, but the tooth of one or other row in advance 

 of its mate, the articular border showing obliquely truncated facets 

 at one or other angle by which it is coadapted to the articular faces 



*Faune du Calcaire Carbonifere de la Belgique, 11. p. 38, 1878. 



tReport on the Geology of Londonderry, etc. by J. E. Portlock, F. B. 8., F.G. S., etc., 

 1843, p. 462. 



