THE DERMAL FIN-EAYS OF FISHES. 477 



included in the bony snbstance forming the lepidotrichiaj yet 

 they cannot be said to give rise to the latter by coalescence or 

 growth.^ In fact, the actinotrichia take no important part in 

 the formation of the lepidotrichia, and the two kinds of 

 dermal rays are of distinct origin. The general relations of 

 these structures in the Teleostei are illustrated in Text-fig. 4, 

 p. 488. 



There are many Teleostean fish in which the scaling of the 

 body spreads over the opercnlar region and the skull, to a 

 greater or less extent. That this extension of the scales over 

 bones of dermal origin is secondary can scarcely be doubted. 

 The same tendency for the scales to spread may be observed 

 on the fins, especially the caudal fin, of many Teleosteans 

 (fig. 59). It is carried to an extreme in such cases as 

 Choetodon and Hasmulon,^ where all the fins are covered with 

 minute scales similar in character to those of the body. 

 These small scales lie on the outside of the lepidotrichia, 

 which are normally developed, and to which they bear no 

 direct relation in number or in disposition (figs. CG and 67). 

 The significance of these facts will be further discussed later 

 on (p. 491). 



A m i a. 



In A mi a calva the jointed dermal rays resemble the 

 lepidotrichia of the Teleostei in every particular, both with 

 regard to their structure and development, and with regard 

 to their relation to the endo-skeletal fin-su|)ports. Actino- 

 trichia are also found in the young, and at the edge of all the 

 fins in the adult. 



Lepidosteus. 



The ''bony pike" is provided with lepidotrichia and actino- 

 trichia in every essential like those of the Teleostei. But it 



' I have confirmed this statement on several genera, amongst others on 

 Chsetostomus, a Siluroid closely allied to those studied by Hertwig. 



2 I am much indebted to Mr. G. A. Boulenger for the gift of specimens of 

 Chsttodon and Ilsemulon, and also for many useful suggestions. 



