bo SPATULARIiE. 



which the parietes of the medullary canal have been divided parallel 

 to its course, the calcigerous tubes are cut through transversely, and 

 their arese, from the calcareous nature of their contents, are seen like 

 fine dots. 



SPATULARI^. 



26.Polyodon. — Of the other free-gilled Plagiostomouslishes the Stu- 

 rionidcBOV family of Sturgeons and the Planirostra are edentulous. The 

 Polyodon, which belongs to the same natural family as the Planirostra^ 

 viz : the SpatularicB of Muller, has small recurved teeth on the maxil- 

 lary and anterior branchial cartilages, at least, at the earlier periods of 

 life. The specimens of Polyodon, (e. g. in the Parisian Museum), which 

 exceed three feet in length are, like the Planirostra, edentulous ; but 

 in a Polyodon of the same species about one foot long, there are two 

 row-s of small and slightly recurved teeth in the upper, and a single 

 row of similar teeth in the low^er jaw : similar teeth occur likewise on 

 the two anterior branchial arches, where these join the tongue ; and 

 again on their expanded extremities which are attached to the palatal 

 region of the mouth :(1) here, therefore, there are both maxillary and 

 branchial teeth. Thus the Spatularice not only exhibit a tendency to 

 the structure of the ordinary osseous fishes in the condition of their 

 branchial apparatus, but likewise in that of their dental system ; the 

 Polyodon being the only plagiostomous fish which has both maxillary 

 and branchial teeth. 



CHAPTER IV. 



TEETH OF THE GANOID FISHES. 



LEPIDOIDS. 



27. The consideration of the teeth of this family of Ganoid 

 fishes is not without interest, although as compared with the Pla- 

 giostomes, few modifications require to be noticed. All the repre- 

 sentatives of the family have disappeared from the present theatre of 

 vital phenomena ; yet although the period when they existed is the 



(1) Miiller, Vergleichende Anatomic der Myxinoiden, p. 150. 



