Arthrodires 589 



dental plates * were not fixed in the sense in which these ele- 

 ments are in the true Pisces. On the evidence of several types, 

 Dinichthys, Titanichthys, Mylostoma, Trachosteus, Diplognathus, 

 and other of the American forms, Macropetalichthys f excepted, 

 there is the clearest proof that each element of the jaws had a 

 considerable amount of independent movement. On account 

 of the mobility of these elements the name Arthrognathi is 

 suggested. Thus the mandibular rami could change the angle 

 of inclination towards each other, as well as their plane with 

 reference to the vertical axis. So, too, could the ' premaxillae ' 

 be protracted like a pair of bent fingers, and it is more than 

 probable that the ' maxillae ' had a considerable amount of inde- 

 pendent movement. In connection with these characters it is 

 also important to note that the blades of the 'mandible' show 

 nowhere the faintest trace of an articular facet for attachment 

 to the cranium. In short, the entire plan of the mandibu- 

 lar apparatus in these forms is strikingly unfish-like, although 

 one will frankly confess that it is remarkable that these forms 

 should have paralleled so strikingly the piscine conditions, to 

 the extent of producing mandibular rami margined with teeth, 

 and an arrangement of toothed elements on the 'upper jaw' 

 which resembles superficially the premaxillary and maxillary 

 structures of teleostomes, or the vomero-palatine structures 

 of lung-fishes and chimaeras. 



' ' In the matter of paired fins there seems little evidence to 

 conclude that either pectoral or pelvic fins were present. In 

 spite of the researches upon these forms during the past half- 

 century, no definite remains of pectoral fins have been de- 

 scribed. The so-called pectoral spines described for Dinich- 

 thys by Newberry, whatever they may be, certainly are not, 



* It will be recalled that there is no ground for concluding that the "man- 

 dibular rami' possessed an endoskeletal core, and were comparable, there- 

 fore, to the somewhat mobile jaws of Elasmobranchs. On the other hand, 

 there is the strongest evidence that they are entirely comparable to adjacent 

 dermal plates. Histologically they are identical, and in certain cases their 

 exposed surfaces bear the same tuberculation. 



t The similarity of Macropetalichthys to Dinichthyids in the general matter 

 of the dermal plates is so complete that I have had no hesitation in associating it 

 with the Arthrognaths. (Cf. Eastman.) The circumstance that its "jaws" 

 have not yet been found has to a large degree been due to the lack of energy 

 on the part of local collectors. In the corniferous quarries near Delaware, 

 Ohio, this fossil is stated to be relatively abundant. 



