4 



STRUCTURES OF ARTHRODIRAN 



135 



Arthrodirans. Sensory canals have been described chan- 

 nelling the surface of the dermal plates of the dorsal side. 

 In the body region of Coccosteus evidence of a lateral line 

 occurs (Smith Woodward) in a white calcified band fossilized 

 in the region of the space of the notochord. In this form, 

 too, an endoskeletal plate is known, (Fig. 130, U) occurring 

 in the median line in the region of the vent, which must 

 be regarded as " suggesting an internal element of support 

 occurring in the vertical septum between the right and 

 left halves of some paired organ (S. IV.)." The character 

 of the dermal investiture of the trunk has apparently 

 not been described ; it may therefore be of interest to 

 note that the museum of Columbia College has recently 

 acquired two of the hinder dorsal plates of Dinichthys 

 which clearly indicate the presence of integument. The 

 plates are covered by a crinkled epidermis, whose irregular 

 surface traceries resemble the roughened finish of Turkey 

 morocco. This leather-like surface is seen to have been 

 continued over the margin of the plates along the side 

 of the trunk ; traces of scales or tubercles are altogether 

 lacking, and its appearance suggests that it may have been 

 degenerate in structure. 



Among Arthrodirans there occurs a series of most inter- 

 estingly evolved forms ; and it is found more and more 

 evident that they, with other lung-fishes, may have repre- 

 sented the dominant group in the Devonian period, as 

 were the sharks in the Carboniferous, or as are the 

 Teleosts in modern times. There were forms which, 

 like Coccosteus, had eyes at the notches of the head 

 buckler ; others, as Macropetaliclitliys, in which orbits 

 were well centralized ; some, like Dinichthys and Titan- 

 ichthys, with the pineal foramen present ; some with 

 pectoral spines (?) ; some with elaborately sculptured derm 



