THE SKULLS OF AMPHIBIA. 217 



This arch clearly answers to the sub-ocular arch of the 

 Lampreys and to the sub-ocular cartilage of the Chimaeroids 

 and Lejndosiren, and corresponds with the palato-quadrate, 

 hyomandibular, and symplectic cartilages of the embryonic 

 osseous fish taken together. The distal end of this cartila- 

 ginous pedicle commonly presents a larger or smaller ossification 

 of its substance, which represents the quadrate bone. Now, 

 the problematical bone (z) lies on the outer side of the pedicle, 

 and I was at one time inclined to think that it represented the 

 hyomandibular bone of osseous fish — being largely led to that 

 impression by the great size of the hyomandibular and the 

 comparative minuteness of the quadrate in the Conger and the 

 Murcenoid fishes. But the hyomandibular is an ossification 

 in the cartilage of the suspensorium, not a membrane bone. 

 The bone has been compared with the tympanic, but the 

 tympanic membrane has a special and distinct supporting ring 

 in the Frogs. It has been identified, again, with the squamosal, 

 but it lies too far down on the outer side of the pedicle for that 

 bone. Tracing the changes of form in this bone (which is very 

 constant in the Amphibia) downwards to the Menobranchus 

 and Siren, its resemblance in these perennibranchiates to the 

 bone (F) of Lejndosiren becomes very striking ; and I am dis- 

 posed to identify it with that bone, which, as I have stated 

 above, has much resemblance to the pre-operculum of osseous 

 fishes. 



The mandible of Amphibia is commonly composed of three 

 pieces — a dentary, an angular, and an articular. The latter, 

 always continuous with Meckel's cartilage, may itself remain 

 persistently cartilaginous. 



The skull of the tadpole, before ossification has commenced, 

 presents a cartilaginous base, in which the notochord terminates 

 in a point, immediately behind the pituitary fossa. At the sides, 

 the basal cartilage expands into two oval auditory capsules, and 

 in front passes into the trabecular cranii, which embrace the 

 membranous floor of the pituitary fossa, and reunite in the 

 broad ethmo-vomerine cartilage. The .apex of a sub-ocular arch, 

 connected, behind, with the auditory region of the basis cranii, 

 and, in front, with the prefrontal region, furnishes an articular 



