FISHES OF THE CARBONIFERCUS SYSTEM, 179 
of the objects which are described by McCoy under the name of Petrodus 
and now thought to be dermal tubercles. Yet they are so large and mas- 
sive that they present in this respect little similarity to any hitherto known 
tubercles, and one of the specimens before us has the upper surface pecu- 
liarly flattened as though by wear. Hence I have thought best to consider 
them the crushing teeth of Elasmobranch fishes, and perhaps allied to Helo- 
dus, as defined by Agassiz and McCoy. 
Since the above description was written a large amount of additional 
material bearing on this subject has been obtained by Prof. William Kepler, 
of Berea, Ohio, who has kindly put it into my hands for study. This mate- 
rial consists of a number of the teeth of Mazodus and the jaws which sup- 
ported them. They were found dismembered, but in close proximity, and 
intermingled so that there can be no reasonable doubt that they belonged 
to one individual, and they throw much light on the structure and relations 
of the genus. The jaws were composed of prismatic cartilage, which in 
virtue of the special duty they performed were more ossified than other 
portions of the skeleton, and hence were firmer and less perishable. I have 
frequently found this condition of preservation in. the jaws of Diplodus 
in the Coal Measures and Cladodus in the Waverly shales, the rami sep- 
arated, the teeth sometimes in position, oftener scattered about in imme- 
diate proximity to them. As in all such cases, the jaws are flattened 
and distorted, but retain much of their substance and approximately their 
outlines. 
' The mandibles of Mazodus are seven to eight inches long and an inch 
and a quarter to one and a half inches wide. They show on the upper sur- 
faces depressions which correspond to the position of the teeth. Of these, 
ten were found grouped together, all in contact and some of them in their 
relative positions. A larger number evidently composed the dental series, 
but the others were lost. Those preserved show a great diversity of size 
and form, the larger and more angular teeth having several smaller ones, 
and these more elliptical in shape grouped around them. How many com- 
posed the complete dentition we have as yet no means of knowing, but 
those found in this group apparently form but a part of the set belonging 
to the under jaw. 
