PLAGIOSTOMES. 25 



in Centrina, and Squatina, there are two on each side of the upper, 

 and one on each side of the lower Up ; these are figured at a & and c, 

 PL 10, fig. 2, in the monk-fish. In the Narcine or Brazilian torpedo, 

 distinct labial cartilages are associated with the dentigerous maxil- 

 lary arches, and also with the palatine and pterygoidean cartilages ; 

 Professor Miiller has, therefore, rejected the interpretation of Cuvier, 

 according to which, the anterior or superior dentigerous arch of the 

 plagiostomes is the homologue of the palatine bones, and the posterior 

 one, the homologue of the post -mandibular element of the lower jaw, 

 the intermaxillary, maxillary, and premandibular bones being repre- 

 sented by the edentulous labial cartilages. This interpretation, 

 besides being invalidated by the anatomy of the Narcine, also involves 

 the anomaly of the teeth being developed on the articular, or post- 

 mandibular element of the lower jaw, where they are never situated 

 in any other vertebrate animal. A more extended comparison than 

 Cuvier had the means of instituting, and especially a study of the 

 structure of the cranium of the Cestracion, in which the labial car- 

 tilages have disappeared, and the development of the dentigerous 

 arches have advanced nearer to the osseous tj^pe than in other 

 plagiostomes, clearly prove that the dentigerous cartilaginous arches 

 of the sharks and rays represent, the one, the combined maxillaries 

 and intermaxillaries, the other, the confluent articular and dentary 

 elements of the lower jaw. 



The teeth are not immediately connected with these cartilagi- 

 nous arches ; no cartilaginous fish has teeth implanted in maxillary al- 

 veolar cavities, or confluent with the substance of the jaw even when 

 the external crust is ossified, but they are always attached as already 

 stated, to the fibrous and mucous membranes which cover the maxillary 

 cartilages ;(1) hence, it occurs in certain genera, as Myliohates and 

 Scymnus, that a single tooth in the median plane may lie directly 

 across the symphysis, and be supported by the two rami of the jaw. 

 The plagiostomes, like many other natural famihes of fishes, pre- 



(1) Any organic fossil which exhibits a tooth implanted by two fangs in a double 

 socket must be mammiferous, since the only fishes' teeth which approach such a tooth in form 

 are those with a bifurcate base, belonging to certain sharks, while the socketed teeth of reptiles 

 have only a single fang. 



