1884, ] 573 (Cope, 
anguineus, whose teeth, as represented, do not differ generically from those 
of Diplodus. This is an interesting discovery, indicating that this genus, 
and not Ceratodusa, is the oldest type of vertebrate now known in the liv- 
ing state. : 
My collections from the Permian beds of Texas include not only 
numerous teeth, but jaws and crania, Among these I recognize two types 
of teeth, which I cannot distinguish from those of the D. compressus 
Newb., and D. gibbosus Agass. Whether these species belong to the same 
genus, is a question, which I will discuss at the close of this article. I pro- 
visionally refer the D. compressus to a distinct genus, Didymodus, and 
will so call it in this article. 
The determination of the characters of this genus is a point of much 
interest The teeth resemble those of the existing sharks more than do 
those of any other genus of the Paleozoic ages, but the antecedent im- 
probability of the modern type having existed at such an early period of 
the earth’s history, is shown to be well founded by the present investiga- 
tion, which also throws much light on the question of the general phylo- 
geny of the fishes. 
I. DxEscriprion. 
Twelve more or less complete crania of species of Didymodus are in my 
collection, and one set of jaws with small teeth and part of the cranium 
attached. One of the crania, unfortunately much broken, exhibits also 
some large teeth. All were found by the late Jacob Boll in the Permian 
beds of Texas. 
The skull of this species forms a continuum, which, however, displays 
distinct segments. First, however, as to the tissue of which it is composed. 
Both on the surface and in transverse fractures, it is more or less finely 
granular, the granules distinctly visible to the naked eye. These granules 
are composed of gypsum, as is also the matrix of a darker color in which 
they lie imbedded. Two hypotheses may be entertained regarding this 
structure. Hirst, These granules may be regarded as the casts of coarse 
cartilage cells, and the matrix be in the place of the intercellular cartilage, 
replaced like the woody tissue in petrified wood. Second, The granules 
may be looked upon as replacements of osseous granules, such as cover 
the chondrocranium of most Elasmobranch fishes, while the matrix may 
be a replacement of the cartilage. The latter hypothesis is the more 
_ probable of the two, for two reasons: First, There is little probability of 
an unsupported chondrocranium retaining its form sufficiently long to per- 
mit the filling of its cells with a mineral deposit. Second, The granular 
type of ossification is well known in existing Elasmobranchs. It is only 
necessary to believe that the chondrocranium is penetrated by this kind 
of ossification. This state of things exists in the jaws also, which I de- 
scribe later. This structure has already been observed by Kner in the 
genus Pleuracanthus. 
The osseous cranium is abbreviated anteriorly, and elongated posteriorly, 
