On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 331 
The Plagiostomatous fishes may be conveniently divided into six groups for the 
purposes of the present work, viz.:— 
I. Hybodontidee.—Type, Ctenacanthus. 
II. Orodontidze.—Type, Orodus. 
III. Petalodontide.—Type, Petalodus. 
IV. Cochhodontidee.—Type, Cochliodus. 
V. Psammodontidz.—Type, Psammodus. 
VI. Copodontidee.—Type, Copodus. 
The first group (Hybodontidze) comprises the spines of Ctenacanthus and probably 
some others. From the great resemblance of Ctenacanthus with Hybodus it has 
been recognised by the late Sir P. Egerton and others as a Hybodont and arranged 
accordingly, The teeth of Hybodus are well known, many specimens having been 
discovered in the Lias of Lyme Regis, which have proved conclusively their connec- 
tion with the spines. The teeth and spines of Ctenacanthus have not been so found, 
but judging from analogy it appears probable that the teeth of Cladodus may have 
been associated with the spines of Ctenacanthus. Except that the Cladodi are more 
formidable and that the coronal prominences are more prominent, they bear a close 
resemblance to the teeth of Hybodus. 
The Orodontidze comprise a group of teeth related to the existing Cestracionts, 
but in several essential respects differing from them, as will be stated more fully 
hereafter. 
The Petalodontidze will embrace the genera defined by Messrs. Newberry and 
Worthen in the synopsis of the Petalodont genera (“ Geol. Survey, Illinois,” Vol. IL., 
p- 31) with the addition of some genera not included by those authors. | This group 
has generally been considered by various authors as more or less associated with the 
family of Cestracionts, but, as will be seen, there are reasons why it should be 
removed from this association, and may be considered as possessing sufficiently 
distinct and characteristic features to render necessary the constitution of a separate 
family, as indicated by Messrs. Newberry and Worthen. ‘The peculiar arrangement 
of teeth and their apparent relationship to Janassa of Miinster also confirms this 
separation, and may indicate an evolutional passage between the sharks and the 
rays. The large and beautiful series of the teeth of the Petalorhynchus in the 
collection of the Earl of Enniskillen have rendered possible this generalization, and 
to Lord Enniskillen is due the credit of having suggested the relationship of the 
genus to Janassa. 
The Cochliodontidze embraces an interesting and most peculiar series of forms of 
dentition. In many respects it presents resemblances to the palates of the existing 
Cestracion, but it is difficult to conceive that the numerous teeth of Cestracion 
should be very closely related to the single enrolled tooth of some Cochliodonts. 
The point of resemblance consists in the cartilaginous jaw being enveloped in each 
