204 PALAEONTOLOGY OF ILLINOIS. 



The above noticed forms are about equally represented in num- 

 bers in the collections from the Upper Burlington "fish- bed," where 

 they are not uncommon fossils. The truncated articular facets vary 

 somewhat in extent as they do also in position, occurring in one 

 individual at one angle and in another at the opposite angle ; and 

 judging from the character of the articular border, it seems most 

 probable that the teeth were ranged in double rows, the individuals 

 of one or the other range slightly in advance of the corresponding 

 teeth of the opposite range, the long articular face and short facet 

 of one tooth joining the same parts of the opposite contiguous tooth. 

 This presumed disposition of the teeth produces the alternating ap- 

 pearance shown in the outline diagrams given in connection with 

 the illustrations of the several sorts of teeth here specifically asso- 

 ciated. The individuals of the two forms show precisely the same 

 variability in the truncated angle of the inner articular border, though 

 the asymmetry is much less pronounced in the supposed maxillary form 

 than it is in the shorter form referred to the mandible of the same 

 species. Indeed, it is the absence of symmetry in these teeth that 

 offers one of the strongest contrasts with the living representatives of 

 the family. Yet it should be remembered that the existing Myliodonts 

 are subject to abnormal variations, such as the interpolation of an 

 extra row of lateral dental plates, while the individuals of the sev- 

 eral ranges are placed in alternating order. The collections have 

 been ransacked, but without result in revealing the vestige of a sym- 

 metrical median dental plate, such an one as would restore to the 

 rows of teeth a perfectly symmetrical arrangement, like that illus- 

 trated in the hypothetical diagram annexed, the lateral forms of which 



will readily be recognized in the 

 illustrations of actual specimens^ 

 We can, therefore, only conjecture 

 the possible existence of median 

 teeth similar in shape to the sup- 

 posed form shown in the diagram, 



Hypothetical diagram showing median ov n 4.1 ok < n-i/-v>rro4- Via 



dental plates of Psam. Spring eri (mandible), and. tiieir absence amongst W6 



score and a half of examples of the lateral forms contained in the 

 collections, would seem to afford slight grounds for the supposition 

 of the existence of intermediate symmetrical teeth in the dental 

 formula of the genus to which the species belongs. 



