SIRENOIDEI. 61 



roof cannot be paralleled by the normal arrangement observed 

 among Teleostomi; the dentition has always been confined to 

 the inner bones of the mouth ; and the fin-rays have never 

 advanced in development beyond the delicate filaments (actino- 

 trichia) which cluster round the supports. Moreover, the 

 supports of the median fins have always remained equal in 

 number to the apposed neural and haemal arches of the trunk, 

 except where these fins have undergone subdivision. 



ORDER 1. SIRENOIDEI. 



The typical Dipnoi of the Sirenoid order, represented at 

 the present day by Lepidosiren, Protopterus, and Ceratodus 

 (Epiceratodus), have numerous Palaeozoic representatives which 

 only appear to differ from the modern types in three essential 



Fio. 48. 



Dipterus valenciennesi ; roof of skull, imperfect at the sides, and snout pointing 

 upwards, about nat. size. L. Old Red Sandstone; N. Scotland. (After 

 Pander.) 



particulars. In these old fishes the cranial roof-bones are small 

 and numerous (fig. 48) ; the median fins are often subdivided ; 

 and the dental plates nearly always exhibit more or less clearly 

 the points of the originally-separate denticles of which they are 



