VEETEBKATES. 143 



generic identity of the forms here referred to the posterior position 

 upon the upper and lower jaws respectively, to which he then gave 

 the name Deltodus, which was accepted by European authorities 

 and recognized in various publications. Prior to the above date, 

 Professor Frederick McCoy, in the British Palaeozoic Fossils, 1855, 

 recognized the probable identity of the narrow median teeth, here 

 referred to the mandibles, with the so-called "anterior teeth" of 

 Deltodus subltevis, which he described under the name Pcecilodus 

 parallelus. The latter form presents the same general figure and 

 coronal contour here ascribed to the median form, although at the 

 same time it presents marked specific characters. Heretofore 

 authors have not essayed the distinct definition of the dental ele- 

 ments pertaining to the upper and lower jaws of this genus, and in 

 the attempt in that direction here initiated we are profoundly 

 conscious of the meagre, imperfect data from which to draw evidence 

 which shall not be open to objection on account of its inconclusive 

 nature. However the facts are briefly stated, and if they do not 

 carry conviction as to the justness of the interpretations arrived at 

 they will at least show the difficulties in the way of the student 

 who shall attempt the restoration of extinct forms from few and 

 detached remains such as are represented by these teeth. 



We are thoroughly satisfied with the conjecture of Professor 

 McCoy as to the relative position and specific identity of the narrow 

 median tooth, as above pointed out; but, on the other hand, the 

 data do not sustain former inferences as to the position occupied 

 by the form with which the narrow teeth were associated upon the 

 jaw. The so-called "anterior" tooth possesses essentially the char- 

 acteristics of the form which, in Cochliodus, is known to hold a 

 posterior or terminal position upon the mandible. The latter form 

 thus transposed readily lends itself to interpreting the median posi- 

 tion of the succeeding narrow tooth, the articular walls exactly con- 

 forming in coadaptation, and thus placed they assume a symmetrical 

 disposition perfectly in consonance with that characterizing the man- 

 dibles of Cochliodus, P<zcilodus and Cestracion; on the other hand, it 

 is impossible to conceive of so intimate relations in the case of the 

 so-called "terminal" tooth with either of the forms just alluded to. 

 However, certain features are noticeable by which a certain striking 

 resemblance is manifest between the median form and that here 

 referred to the posterior or terminal position on the upper jaw (the 

 "terminal" tooth of authors). This consists in the remarkably close 

 similarity in coronal contour which obtains in either form alike, 



