Eastman — Dipnoan Affinities of Arthrodires. 141 



Space is wanting to enter into detailed comparisons of the 

 cranial rooting plates as they occur in Arthrodires and modern 

 Dipnoans, but it will be evident from the annexed figures that 

 there is marked agreement between them. Allowance must of 

 course be made for the fact that some of the elements, such as 

 the pre- and sub-orbitals, remain more or less cartilagi- 

 nous in the modern form. The centrals, also, have become 

 enlarged, much more so than in the Triassic C. sturii, and 

 excluded from contact with each other in the median line 

 though elongation of the median occipital. Yet the latter 

 element is relatively less elongated than in Homosteus, and 

 Macropetalichthys affords a parallel example of fusion 

 between the pineal and rostral. Turning to Protopterus, we find 

 that the bones corresponding to the centrals are actually in 

 contact for a certain distance anteriorly, and those of C. stu- 

 rii have practically the some conformation as in Dinichthys. 

 In all known Dipnoans, recent and fossil, two opercular bones 

 are present, but these remain for a time fused together in the 

 young Neoceratodus. Coccosteus, and Dinichthys as well, is 

 commonly understood as having one opercular element ; Jae- 

 kel, however, affirms the existence of two in the former genus. * 

 The significance of this observation, if confirmed, is apparent. 



Axis and B ody- armoring . — Both in living Dipnoans and 

 amongst all Arthrodires where the vertebral column is known, 

 the latter remains notochoral, and the neural and haemal arches, 

 together with the dorsal fin-supports, have expanded extremi- 

 ties. The resemblance between Coccosteus and Neoceratodus 

 in this respect is very great. The encasement of the anterior 

 portion of the trunk in dermal armor is to be looked upon as 

 a specialized feature peculiar to Arthrodires, yet comparable, 

 in a general way, to the extensive ossification observed amongst 

 Ctenodipterines, and to the ganoine investment of their squa- 

 mation. The question of body-armoring is of purely second- 

 ary importance in determining affinities, inasmuch as wide 

 variation prevails amongst closely related forms. So far as 

 we may rely on negative evidence, Macropetalichthys and 

 Asterosteus were unprotected by abdominal armor, and within 

 a single family of Ostracophores, Pterichthys is scaled, Both- 

 riolepis naked. The fact that two of the dermal covering 

 plates are articulated with the heaclshield should occasion no 

 surprise, when it is remembered that the so-called "cranial 

 ribs " — which may represent morphologically a pair of costal 

 elements — articulate with the skull in modern Dipnoans. 



Fins. — Many writers have taken it for granted that the tail 

 of Coccosteus was heterocercal. Not a particle of evidence 



* Jaekel, 0., Ueber Coccosteus und die Beurtheilung der Placodermen, 

 Sitzungsber. Gesellsch. Naturf. Freunde, Jahrg. 1902, p. 109. 



