424 COMPARATIVE ANATOMY. 



papilla, whence fine branched canals radiate out to the surface. 

 The placoid scale has therefore the structure of dentine, is covered 

 by enamel, and is continued at its base into a plate formed of 

 osseous tissue ; as they agree with teeth in structure they may be 

 spoken of as dermal denticles. In the Rays these structures have 

 altogether disappeared (Electric Ray), or are replaced by larger 

 structures, which are grouped together in the form of spines or 

 larger bony teeth, or are separate from one another (Spiny Rays). 



The dermal denticles of the Shark are very generally converted 

 into larger bony plates in the Gano'idei ; in the Rhombifera they 

 have not only the same arrangement on the body, but have essen- 

 tially the same minute structure. In the Sturiones larger bony plates 

 alternate with smaller ones. They generally retain exactly the 

 rhomb-form, which is lost in the rest of the Gano'idei (the Cyclifera). 

 The common flat and thin scales of the Teleostei follow on here. 

 They appear to differ in many points from the ganoid scales, and re- 

 present an offshoot from the type, which obtains in the Gano'idei, and 

 which may be derived from the Selachii ; this offshoot is characterised 

 by its variety of form. 



In many Teleostei the scales undergo complete degeneration. 

 On the other hand they give rise to parts which differ somewhat 

 from scales, and which are formed by the fusion of the dermal 

 denticles ; such are the plates and spines of the Plectognathi, where 

 the plates may become more firmly united together and form a 

 connected carapace (Ostraeion, Lophobranchii). 



Parts which are likewise formed from the concrescence of dermal 

 denticles are found in the integument which covers the appendages 

 of the GanoTdei and Teleostei. To compensate for the atrophy of the 

 internal or primary skeleton of the limbs, these bony plates form a 

 number of rays, which often branch dichotomously at their ends, and 

 unite to form an organ of support for the fins (secondary skeleton of 

 the fin). The ray which occupies the anterior edge of the fin is 

 frequently massive, or gives rise to a strong spiny ray, which may be 

 connected with the internal skeleton. This ray may not only be 

 larger than the rest of the rays, but it may even, as in certain 

 Siluroids, represent the whole of the pectoral fin. 



Hertwk;, 0., Ueber d. Baa u. die Enfcw. der Placoi'dsclmppen u. der Ziihnc der 

 Selacliicr. Jon. Zeitschr. Bd. VIII. — The same, Uebcr das Hautskelet der 

 Fische. Morph. Jahrb. II. 



§ ;325. 



The ossifications of the integument are of special importance in 

 those regions of the body where parts of the internal skeleton come 

 to the surface. These ossifications are developed in just the same 

 way as the bony plates on other regions of the surface of the body, 

 and may likewise be derived from the indifferent stage represented 

 by the dermal denticles. Although the various kinds of dermal bones 

 which are found on the trunk have an importance which is limited to 



