FISHES OF THE GENUS ACENTROPHORUS. 



39 



also by at least one fact of fossil ontogeny, A young example of 

 Dapedius, 9 cm. long and beautifully preserved (text-fig. 16), 

 which has been kindh^ lent me by Prof. D. M. S. Watson, has a 

 caudal lobe extending just two-thirds the length of the upper 

 border of the tail, instead of only one-third as in the adult ; and 

 for some distance towards its apex the lobe is marked by a single 

 line of very small and narrow scales. 



The head of AceniropJiorus also, though in side view almost 

 entirely typical of a Semionotid, shows when seen from above 

 (text-fig. 3) some decided primitive traits. The shape of the 



Text-fififure 16. 



Dapedius orbis. Tail of a young example, total length 9 cm., showing extended 

 caudal lobe. Lower Lias ; localitj' uncertain. D. M. S. Watson Collection. 

 About four times natural size. 



frontal shield and of the supratemporals, and the large leaf- 

 shaped post-temporals, are all strongly reminiscent of Palceoniscus. 

 So also is the internal pattern and articulation of the scales, 

 especially in the earliest species, A. glaj^hyrus. 



On the other hand, there are certain points in the structure of 

 Acenfrophorus which seem to be peculiar to this genus among the 

 Semionotidae. The orbit is very large and encircled by only a 

 single chain of bones ; the. scales, compared Avith those of other 

 Semionotids, are thin, deeply overlapping and of unusual range 

 of difference in size, and do not produce the typical " tessellated 



