EASTMxVN : STRUCTURE A2^D RELATIONS OF MYLOSTOMA. 13 



N"eoceratodus the median series is reduced to one anterior element, 

 covering the pineal and rostral (or ethmoid) regions, and one elongated 

 posterior element (M 0), these two plates being suturally united with 

 each other, and excluding the paired central elements from contact 

 along the median line. Nevertheless in Protopterus the plates corre- 

 sponding to the centrals are actually in apposition along the median 



Fig. B. — Neoceratodus forsteri (Krefft). Dorsal aspect of cranial roof, drawn as if 

 flattened out, and dermal plates lettered to correspond with those in Dinichthys. Cartilag- 

 inous elements (PC) dotted ; suborbitals omitted. From a specimen in M. C. Z., x J. 



line for a certain distance anteriorly, and in Homosteus the median 

 occipital is relatively more elongated than in Neoceratodus. The latter 

 shows an abrttpt downward deflection of the bone-substance along a 

 portion of the posterior margin of the cranial roof. Like conditions 

 are found in Macropetalichthys, even more conspicuously developed. 

 Macropetalichthys and Homosteus both have the external occipital 

 greatly enlarged at the expense of the central elements. Neoceratodus, 

 on the other hand, has the centrals enlarged at the expense of the 

 external occipitals (Fig. B, EO). In Ceratodus sturii these plates 

 are more nearly as in Dinichthys. 



The remaining elements of the cranial roof in Neoceratodus are easily 



