The Class Elasmobranchii or Shark-like Fishes 521 



The body is slender, not depressed, and probably covered with 

 smooth skin. The teeth have two or more blunt cusps, some- 

 times with a smaller one between and a blunt button behind. 

 The interneural cartilages are more numerous than the neural 

 spines. The genera are imperfectly known, the skeleton of 

 Pleuracanthus decheni only being well preserved This is the 

 type of the genus called Xenacanthus which, according to Wood- 

 ward, is identical with Pleuracanthus, a genus otherwise known 

 from spines only. The denticles on the spine are straight 

 or hooked backward, in Pleuracanthus (l&vissimus), the spine 

 being flattened. In Orthacanthus (cylindricus) , the spine is 

 cylindrical in section. The species called Dittodus and Didy- 

 modus are known from the teeth only. These resemble the 



FIG. 310. Shoulder-girdle and pectoral fins of Cladodus neilsoni Traquair. 



teeth of Chlamydoselachus . It is not known that Dittodus pos- 

 sesses the nuchal spine, although detached spines like those of 

 Pleuracanthus lie about in remains called Didymodus in the 

 Permain rocks of Texas. In Dicranodus texensis the palato- 

 quadrate articulates with the postorbital process of the cranium, 

 as in the Hexanchidce, and the hyomandibular is slender. 



A genus, Chondrenchelys, rom the sub-Carboniferous of 



