A. Smith Woodward—On a Species of Onychodus. 501 
and so rendered its minute structure to a certain extent recognizable. 
The base is conspicuously vascular, except in a thin, sharply-defined 
layer forming the concave margin; and each tooth is likewise con- 
stituted mostly of vascular tissue, with a thin, dense outer layer. 
A very small cavity also appears to occupy the centre of each 
dental cone. 
The genus Onychodus was originally founded by Prof. Newberry 
upon detached teeth similar to those of the Ledbury fossil, dis- 
covered in the Corniferous Limestone of Ohio. These teeth were at 
first supposed to be referable to the great Placoderm, Macropetal- 
ichthys, occurring in the same beds; but their discovery soon after- 
wards in connected series naturally led to the suspicion that they 
might rather pertain to a Selachian. Still later, the problematical 
dentition was found associated with cranial plates, tooth-bearing 
maxille and dentary bones, and numerous round scales; and in 
1878, complete proof of its exact position in the jaws of the original 
fish had at last been obtained. It appears that the slender bony 
arch is a mesial element fitting in a well-marked groove at the 
symphysis of the dentary bones; while the long, sigmoidally-curved 
teeth project directly forwards in front. The head-bones are 
numerous, and the large round scales are deeply overlapping, being 
ornamented much like those of Glyptolepis ; whence Prof. Newberry 
concludes that Onychodus was most probably one of the great group 
of Crossopterygidee. 
The fossil now under discussion is thus referable to a “‘ Ganoid,” 
and it evidently represents the presymphysial bone of some later 
genera (e.g. Aspidorhynehus and Belonostomus), even if it be not 
altogether homologous with that element. The appearance of the 
series of teeth is at first suggestive of a dental succession like that 
of Selachians; and in this connection it would be interesting to 
know precisely the histological characters of the fossil. Though 
comparatively abundant, none of the American examples seem to 
have been yet examined microscopically ; but it is to be hoped that 
some such investigation may soon be made—more especially as 
comparisons have already been suggested with the problematical 
Carboniferous Hdestus.? 
The coiling of the inferior extremity of the base in the Ledbury 
fossil has not hitherto been noted in the American specimens; and 
this may be either a normal feature, or merely a post-mortem 
accident to a possibly pliable tissue. Other differences, however, of 
at least specific value, are also observable between this and the 
described American types of “intermandibular arch”: the new 
fossil is much smaller than those already known, and the size of the 
teeth is apparently less uniform. Awaiting further evidence, there- 
fore, for more precise definition, the species of the Ledbury Passage 
Beds may receive the name of Onychodus anglicus. 
1 Fanny R. M. Hitchcock, ‘‘On the Homologies of the so-called Spines of 
Edestus,’’ Proc. Amer. Assoc. Ady. Sci. 1887. See also J. S. Newberry, ‘‘ On the 
Structure and Relations of Hdestus,” Ann. New York Acad. Sci. vol. iy. (1888), 
p. 116. 
