632 J. W. DAVIS ON REMAINS OF FOSSIL FISHES 
This example is unique. in Mr. Horne’s collection. Its relation- 
ship to other groups of fish-remains is extremely problematical. 
There is no genus hitherto described from the Mountain Limestone 
to which it appears to possess affinity. The teeth of the genus Peri- 
plectrodus, St. J. & W. (Paleontology of Illinois, vol. vi. p. 324), 
trom the Lower Carboniferous beds of Iowa and Illinois, are de- 
scribed as being symmetrically inrolled from within outward, the 
crown consisting of a transverse series of strong median cusps, 
flanked by at least one row of small denticles on each side, which 
regularly increase from the outer to the inner extremity. The sym- 
metrical or exact vertical inrolment in Periplectrodus, the authors 
consider would indicate that the teeth held an isolated position, 
either constituting the sole dental element of the fish by which they 
were possessed, or restricted to some part of the mouth other than 
the maxillary elements; or, if they were associated with other teeth, 
that they may have occupied a symphysial position upon the jaws. 
The genus now described offers some points of resemblance to 
Periplectrodus, for, in addition to the cusps or denticles occupying 
the central line of the tooth, itis probable that others existed on 
the lateral portions, their position being marked by the circular 
depressions already mentioned. There is also some indication that 
a fourth cusp existed in a line with the central row, which when 
present would have extended in a more or less horizontal. direction 
as compared with those still present. Whilst agreeing in these 
particulars with the American genus, it differs in one or two 
important particulars. The rounded basal portion does not possess 
the convoluted character of Periplectrodus, whose inrolled form 
appears to partake very much of the character of the second or 
smaller tooth of the lower jaw of Deliodus sublevis, Ag. (see Trans. 
Royal Dublin Soe. n. s. vol. i. pl. li. fig. 9a), or of the middle tooth 
of Cochliodus contortus, Ag. (op. cit. pl. li. figs. 5, 5a), in the manner 
of its growth. Messrs. St. John and Worthen state, after the 
examination of a large number of specimens, that the teeth of 
Periplectrodus increase in size with age, and that the cusps on the 
earlier growth of the dental plate become hidden from view by being 
inrolled and- enveloped by the older extremity of the dental plate. 
Excepting the fact that these teeth have longitudinal series of cusps 
or denticles, they do not differ very much from the teeth of the 
Cochliodonts named above; and may possibly have been some- 
what nearly related to them. The tooth from Mr. Horne’s collec- 
tion exhibits none of these peculiarities; the basal portion is quite 
solid and homogeneous in structure. It also appears to be thinner 
or narrower than that of Periplectrodus. 
Genus Dretacopts, Davis, g. n. 
Teeth. Of medium or small size, twice as long as broad; crown 
divided by a deep sulcus into two unequal parts. Anterior portion 
forming a prominent transverse convexity, more or less pointed on 
the inner margin and expanding outwardly; behind the sulcus 
