On some Little-known Bones of the Mammalian Skull. 371 



In most mammals they are absent, their place being taken by the 

 palatine processes of the premaxilla. In only one mammal besides 

 Ornithorhynchus do I know of them occurring as distinct elements 

 in the adult, viz., the bat Mimojpterus. According to Kitchen 

 Parker many Edentata and Insectivora have in foetal life a pair of 

 " anterior vomers," as he called them, supporting the paraseptals, 

 but these early became anchylosed to the premaxillaries to form 

 part of their palatine processes, though there can be little doubt that 

 they are homologous with the prevomers of Ornithorhynchus. Prof. 

 Cleland informs me that Goodsir was acquainted with a small 

 bone occasionally present behind the premaxillary and in front of 

 the vomer, and that in conversation he spoke of it as " John 

 Arthur's bone." Most probably this was a prevomer which had 

 appeared as an abnormality. Gomphognathus has a well-developed 

 typical vomer and a pair of small prevomers. 



Prefrontal, Postfrontal, and Postorbital. 



These three membrane bones which are all present in many 

 reptilian skulls are not generally recognised as occurring in the 

 mammalian skull, and in the very large majority of cases there 

 is no trace of any of them. In Anomodonts and Therocephalians 

 all three are probably always present, but one is usually very small. 

 The bone which forms the upper part of the postorbital arch and 

 passes back by the side of the parietal was formerly regarded as 

 the postfrontal, but the presence of a small bone in front of it seems 

 to show that it ought to be regarded as the postorbital, the small 

 bone being the postfrontal. In the Cynodonts, though prefrontals 

 and postorbitals are well developed, no postfrontals have hitherto 

 been recognised, but as only very few good skulls have been 

 examined it is probable that, as in the Anomodonts, all three bones 

 will yet be recognised. 



In Tritylodon there seems to be a distinct prefrontal and a bone 

 behind the orbit which may be postfrontal or postorbital. Un- 

 fortunately the only known specimen is very imperfect, and there 

 has been much discussion as to whether it is a mammal or a reptile, 

 so that any evidence from Tritylodon does not at present carry very 

 much weight. I have in a previous paper argued in favour of its 

 being a mammal, and should this prove to be confirmed we shall 

 probably have to admit the prefrontal at least as an element of the 

 mammalian skull. 



In Ornithorhynchus and Echidna there are at least two bones in 

 the brain case which are not usually present in the higher forms. 



