IN THE PABAGUAYAN LEPIDOSIREN, ETC. 367 



(C.) The great development of the splenial bones of the lower jaw for the sole 

 support of mandibular teeth, similar to the palatal teeth of the upper jaw, 

 and their union anteriorly in a stout mental symphysis, to which may be 

 added the absence of dentigerous dentary elements. 



(D.) The presence of only two opercular bones — an operculum and an interoper- 

 culum — and, unless represented by the squamosal, the absence of a distinct 

 preopercular element. 



Apparently the only constant cranial character in which all fossil Dipnoi differ 

 from existing Dipnoi is the multiplicity and almost Acipenseroid arrangement of their 

 cranial roofing-bones l . These bones seem to be most numerous in Dipterus (D. valen- 

 ciennesi). A medio-dorsal longitudinal series can be distinguished, flanked on each 

 side by several rows of longitudinally-disposed lateral plates. The inner lateral row 

 of plates on each side tend to meet in pairs between the median plates, so as to 

 interrupt the sutural continuity of the latter. A similar tendency to alternation may 

 be also exhibited by the lateral series of plates, but their arrangement is far from 

 regular, and is not always symmetrical on opposite sides of the cranial roof. There 

 is a general similarity to Dipterus in the disposition of the cranial plates in other 

 extinct Dipnoi, and it is worthy of note that in Ctenodus the plates become somewhat 

 reduced in number, and two of the median series, the most anterior and the most 

 posterior, become so far enlarged as to be distinguishable as a dermal ethmoid 

 and a dermo-supraoccipital [Fritsch, 9]. But, except in the most general manner, it is 

 practically impossible to correlate any of these plates with the numerically-reduced 

 and more definitely-disposed cranial roof-bones of the average Ganoid or Teleostean 

 skull. To what extent the dermal plates are represented in living Dipnoi it is 

 extremely difficult to say. Dollo [7] regards the sparsely-represented dermal cranial 

 bones of existing types as the remains of the much more numerous plates of the fossil 

 Dipnoi. If this be so, the most anterior of the medio-dorsal series of plates in the 

 latter, and a solitary pair of the lateral plates in the orbital or ethmoidal region, may 

 be represented in Ceratodus, Protopterus, and Lepidosiren by the median dermal 

 ethmoid and the paired dermal ectethmoids or lateral ethmoids respectively. On the 

 other hand, if the " scleroparietal " of Ceratodus is a " tendon-bone," it probably has 

 no counterpart in any fossil Dipnoid ; and as the fronto-parietal of Protopterus and 

 Lepidosiren is situated wholly internal to the jaw-muscles, which could scarcely 

 have been the case with any of the cranial plates of Dipterus and its allies, the same 

 conclusion may be suggested with regard to this bone. 



There are also other differences in cranial structure, which, if not distinctive of all 

 fossil Dipnoi, are nevertheless characteristic of particular families or genera, and, so far 

 as our knowledge at present extends, the more important of these may be briefly reviewed. 



1 See remarks! by Smith Woodward on Ctenodus [45, p. 253], 



3b2 



