1884.] 5 i 6 [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 Ceratodus, 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 Palaeozoic 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. Description. 



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. First, 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, 



