198 DESCRIPTION OF FISHES. 
in size to five inches ; the enamel in most cases is worn away ; when it remians, 
the specimen presents a beautiful black lustre. Professor Owen has observed 
that the dental plates in Myliobates, Aitobates and the allied genera are united 
by sutures, a beautiful adaptation to provide against the shocks and rough usage 
to which they must be exposed ; it is the only instance of this mode of junction 
of contiguous teeth known in the animal kingdom. I have represented on Tab. X., 
XI. & XII, six well-marked species, and I have no doubt several others in my 
collection may be easily defined; the age and size of the fish must be borne in 
mind in making these comparisons, for we find the teeth of the recent Myliobates 
varying in the same species according to those circumstances. I saw in 
Cuvier’s museum in the Jardin des Plantes, an extensive collection of recent 
specimens elucidating this range of variety; it is also exemplified by some of the 
specimens in our own College of Surgeons. In four specimens of Zygobates in 
my cabinet, no two are alike; some vary in the size and length of the principal 
dental plates, but more particularly in the shape and size of the accessory or 
lateral ones. 
Myliobates Dixom. (Tab. X. figs. 1,2; Tab. XI. fig. 14; Tab. XII. fig. 3.) 
Professor Agassiz has done me the honour to give my name to this species ; 
he considers it very different from those figured in his work, and does not hesi- 
tate to distinguish it from all its congeners. ‘The dental plates are very large, 
and the relative disproportion of their dimensions very remarkable, the average 
length of the principal plates being about a fifth of their width ; the lateral plates 
are small, narrowish and elongated. In some specimens there is a strongly 
marked depression or groove near the edge ; this is well seen in fig. 3, Tab. XII. 
The figured specimens are all from the upper jaw. The fine one, fig. 14, is 
from the cabinet of G. A. Coombe, Esq. 
One singular example of part of the dental armour of the upper jaw of a species 
of Myliobutes, nearly allied to, or identical with the M. Dixon, has the small 
lateral plates developed on one side only, the large or principal plates thinning 
off to an edge on the opposite side ; this is doubtless an abnormal or accidental 
variety. 
Myliobates toliapicus. (Tab. X. figs. 3, 4, 5; Tab. XII. fig. 4.) 
This species resembles the recent M. aquila; the difference in the fossil con- 
