On the Fossil Fishes of the Carboniferous Limestone Series of Great Brita. 485 
The teeth comprised in the family Petalodontide as indicated in the list of 
genera giver by Messrs. Newberry and Worthen, “ Geol. Survey of Ill,” Vol. II., 
page 31, present an extremely varied series, though each possesses the well-marked 
features which characterize the family. There is in each genus a concavo-convex, 
more or less sharply pointed petal shaped crown ; it may be broader than long as 
in Chomatodus and several of the others, whilst in Petalorhynchus, though a great 
variety may be observed in the form of the teeth, the prevailing form is that in 
which the crown of the tooth is longer than its breadth. The roots of the several 
genera also present a great variety of forms. In Petalodus and Petalorhynchus 
the root or base is strong, tumid, and equalling the crown in length, whilst in 
Antliodus and Chomatodus the root is very small in proportion to the crown. In 
Polyrhizodus and Dactylodus, the root is divided into a number of radicles. Not- 
withstanding their general arrangement into the groups already named there is 
no distinct line dividing the genera from each other, and intermediate forms may 
be selected which combine some of the characteristics of one genus with those 
of another and form connecting links which renders it almost impossible to break 
up the serics into well defined genera. Messrs. Newberry and Worthen had a due 
appreciation of this difficulty, and remark that ‘if we could have for study the 
entire organism now so dimly shadowed forth, in these numerous, variable, and 
disconnected teeth, we should find in the form, in the complete dentition or in 
other organs—the fins, the spines, the branchial openings, etc., characters by which 
the group of fishes here represented might be arranged in a large number of well — 
defined genera, the community of form and structure which their teeth exhibit 
would probably then be shewn to be a family, and not a generic character.” * 
Having briefly reviewed the classificatory position of the Petalodonts, it is 
proposed now to consider their relationship to other forms, fossil and recent, and 
endeavour to deduce their zoological affinities, for this purpose a great help is 
afforded by the genus Petalorhynchus. 
The collection of Lord Enniskillen includes a large and magnificent series of 
specimens of detached teeth of Petalorhynchus, varying very greatly in form and 
size, so much go that without so large a series for comparison, it would have been 
almost impossible to have comprised all the different forms under one specific head. 
A detailed description of the teeth will be given in succeeding pages, and two or 
three typical teeth are depicted on Pl. LXL, figs. 12, 13, 14. In addition to the 
numerous series of detached teeth, there is in the collection referred to, a number 
of vertical rows or sets of teeth (P}. LXL., fig. 16), and a single example showing the 
horizontal or lateral arrangement (Pl. LXL, fig. 15). The latter example consists of 
the crowns of three teeth, a central or median tooth, and two lateral ones. They 
are undoubtedly in the same relative position they occupied in the mouth of the 
living fish ; the crown of the central tooth is much longer than the ordinary form, 
* “Geol. Survey Illinois, Paleont.,” Vol. II., p. 32. 
