FOSSIL FISHES. 611 



If we carry on the comparison to the case of Coccosteus, we should 

 consider these plates the homologues of the interlaterals of that genus. 

 But in those no sign of sutural separation is indicated in any of the fig- 

 ures or descriptions that are accessible to me. Perhaps the small size of 

 Coccosteus may render this intelligible, as the whole clavicle there does 

 not exceed two inches in length. 



On Plate XXXIX is figured, on the same scale, a smaller plate obvi- 

 ously homologous with that above described, but found by the Rev. H. 

 Hertzer in the Huron Shale at Delaware, O. As the Huron Shale has 

 thus far yielded no specimens of Titanichthys it is reasonable to refer this 

 specimen to Dinichthys Hertzeri, though so little has yet been done in 

 the above named stratum that the reference must be merely provisional. 



Again on Plate XXXVIII is figured on the same scale another ho- 

 mologous plate in the collection of Dr. Clark, only six inches in length and 

 differing ironi both the others. There are the same flat surfaces on one 

 side and the same strong ridge rising into a flange and then becoming 

 separate so as to form a double bone at the thick end. 



It seems likely, therefore, that plates of the same general form and 

 nature composed parts of the skeleton of Dinichthys and of Titanichthys 

 with differences of size and outline betokening distinct species. 



The bone figured on Plate XL, fig. 1, is also provisionally referred to 

 Titanichthys. That it was a median plate admits of no doubt. That it 

 was a ventro-median is almost as certain. It would in that case corre- 

 spond to the " lozenge-plate " of Coccosteus. It is very thick and solid 

 and shows a wide overlapped margin all round. At one end is shown 

 another plate which formed a continuation either forward or backward j 

 fitting into the socket excavated in the larger plate. This would then 

 correspond to the antero-ventro-median of Coccosteus. 



Dentition of Titanichthys. 



Since the publication of the Monograph on Fossil Fishes a consid- 

 erable amount of detail regarding the dentition of Titanichthys has been 

 discovered, and we are now able to represent it much more fully than 

 was then possible. A reference to that work will show that the evidence 

 at hand induced Dr. Newberry to believe that the grooved mandible was 

 covered with horn, as in the turtle, or held bony wedges which had dis- 

 appeared. In addition to this he figures in Plate XLVIII a tooth doubt- 

 fully referred to T. Clarki. The latter of the two structures above men- 

 tioned is now known to have been the actual one, in consequence of the 

 discovery by Dr. Clark of the teeth of this species which are represented 

 in place in Fig. 2 of Plate XLII.* One of these was found as repre- 

 sented, the other is supplied from the evidence given by the jaw. 



*Note. Several slight differences between the specimen here figured and that 

 given in the Monograph will be noted on comparison, but in the present condition 

 of our knowledge they are not important enough to constitute a specific distinction. 



