PISCES. 



975 



that of their teeth and bones, as will be evident 

 from the following analysis made by M. Che- 

 vreul of the scales of a Lepidosteus, of a Cheto- 

 don, and of the Perca labrax, after they had 

 been thoroughly dried by exposure during six 

 weeks to a dry atmosphere. In drying, the 

 scales of Lepidosteus lost 1 1 .75 per cent., of 

 Chetodon 13 per cent., and those of the Perca 

 labrax 1 6 per cent. 



^ Iii a preceding article (see OSSEOUS SYSTEM, 

 Comp. Anat.J we have endeavoured to shew 

 that the scales which invest the exterior of the 

 body constituting the exoskeleton of Fishes, 

 by progressive modifications in their size, tex- 

 ture, and arrangement, are converted into very 

 various organs, namely, the apparently osseous 

 plates that cover Lepidosteus and Ostracion, 

 the formidable prickles that stud the external 

 surface of the Diodons, the opercular flaps of 

 the Sturgeon, and even those of the osseous 

 Fishes; the spines of Ga&terosteun, and those 

 which in Silurus, Ba- 

 listes, and Lop/iius, 

 were likewise proved 

 to belong to the epi- 

 dermic or tegumen- 

 tary system ; lastly, 

 the fin-rays and in- 

 terspinous bones of 

 the vertical fins were 

 found to be derivations 

 from the exoskeleton, 

 instead of being, as 

 they have long been 

 considered, parts ap- 

 pertaining to the en- 

 doskeleton or true 

 osseous system. 



The dental organs 

 of vertebrate animals 

 have very naturally 

 been regarded by the 

 old anatomists who 

 confined their osteolo- 

 gical researches to the 

 investigation of the 

 human skeleton, as 

 forming a part of the 

 bony framework of the 

 body, notwithstand- 

 ing that the teeth in 



Any one who with a little care examines the 

 dental apparatus of Fishes will, however, 

 speedily be convinced that the teeth in common 

 with the epidermic structures above enumerated 

 are all of cuticular origin, their connection with 

 the real osseous skeleton, by their roots be- 

 coming consolidated with certain bones of the 

 mouth or implanted into the jaws, being by no 

 means an essential or even constant circum- 

 stance. 



Every one knows that the skin covering the 

 body of the Skate or Thornback is thickly studded 

 with calcareous spines, some of microscopic 

 size, but others of considerable dimensions. 

 On tracing these cuticular spines towards the 

 mouth they are found, as they pass over the 

 manducatory surfaces of the upper and lower 

 jaws, to become suddenly very much increased 

 in size, and are arranged with such regularity 

 that they constitute a very formidable set of 

 dental organs, consisting of ten or a dozen rows 

 of sharp teeth, which answer every purpose con- 

 nected with the seizing and swallowing of food. 

 These teeth, however, or scales, for such they 

 indubitably are, have no connection with the 

 jaws that support them except through the in- 

 termedium of the cutis or mucous membrane 

 covering the mouth, from which they are deve- 

 loped, and are continually in these Plagiostome 

 genera in progress of formation behind as they 

 are worn away in front, their developement 

 being accomplished in the following manner.* 

 A series of minute and closely aggregated pa- 

 pillifbrm matrices or pulps rise in succession 

 from the mucous membrane behind the teeth 

 already formed, which gradually become ossi- 

 fied by the deposition of calcareous salts in the 



Fig. 509. 



-*, 



-U' 



Skull and jaws of Port Jackson Shark ( Cestracion Philippii), shewing the forms 

 and arrangement of the teeth. 



every particular of their economy were con- peripheral cells and radiating tubes of which 



fessedly very different from any other pieces the pulp consists. 



of the skeleton. * Owen, Odontography, 4to. 1840. 



