404 



H. B. POLLARD, 



In Osleolejns, depicted by Pander, the condition must have ap- 

 proached that of Heptanchus for tlie (lei)ressiou oJ" the nasal plates 

 indicates the existence of the prefrontal pit and the pineal aper- 

 ture is in a similar position to that of Hep tan chus, 

 that is, quite anterior. Iw Folypterus the ethmoidal cartilage has 

 grown backwards and dorsally so that the pineal opening is pushed 

 backwards and no longer has a separate existence being lost in the 

 large supracranial fontanelle. The epiphysis, it may be mentioned, 

 extends far forward (Waldschmidt). When the Stegocephali are 

 reached the pineal aperture is parietal as in Lizards. 



In his great work on the skull of Urodeles Prof. Wiedersheim, 

 fourteen years ago, indicated relationships between the Phanerobran- 

 chiata and Selachii, at the same time pointing out that the Urodeles 

 are by no means a homogeneous group. 



On comparing the primordial cranium of a young Polypterus with 

 that of Urodeles the general resemblance is seen to be so great that 

 an anatomist seeing it alone for the first time would certainly place 

 it among the latter. On closer comparison however it will be found 

 to agree entirely with no one group. General similarity is seen iu 

 the exoccipitals , in the opisthotic (which has hitherto in this paper 

 been generally called petrosum to guard against too close comparison 

 with the opisthotic or intercalare of Teleostci), in the supra- and basi- 

 occipital cartilaginous bauds, in the orbitosphenoids, and in the extent 

 of the supra- and infracranial fontanelles. 



The Phanerobranchiata may be considered most nearly related to 

 Polypterus on account of the greater extent of their primordial skull, 

 the greater prominence of the opisthotic and of the saccular wall. 

 'J'hey differ however considerably in the structure of their nasal capsules. 

 Polypterus has no posterior nares and its nasal capsule is very com- 

 plete. In one specimen , there was a slit in the roof of the nasal 

 capsule, not present in the youngest, which may be considered as the 

 first indication of that incompleteness which is met with in so many 

 of the tailed Amphibians. 



Siren agrees with Polypterus in the presence of a rostral pro- 

 jection (seen again in Salamandra atra) and of thé postorbital process 

 which is a partial supraorbital band (Parker). In Menobranchus there 

 is a curious resemblance in the oval block of cartilage representing 

 the basioccipital. 



In Amphiuma the replacement of the Orbitosphenoid by down- 

 ward processes of the parietal and frontal is carried much further than 



