THE BATRACHIA OF NOETH AMERICA. 243 



tbe fore-legs are thus external the brancliial charabers reinaiu in free 

 comuiunicatiou with the external medium by the slit around tbe base 

 of each fore-leg. These soon close, however, and tbe skin of tbe fore- 

 leg is cut ofl" from that of the body adjacent by a distinct seam, which 

 disappears later. Tbis part of the development of the Salientiais one 

 of the most remarkable histories in the zoology of the vertebrata. 



The skin which covers the fore limb of the advanced tadpole is not 

 a part of the true skin which invests the body, since the branchial cav- 

 ity is inclosed long before the leg appears; but it arises beneath the 

 mucous membrane which lines the branchial chambers. Tbis anomaly 

 is perhaps a case of reversion. The fore-legs of primitive Batrachia 

 were no doubt external, as in salamanders, and they became inclosed 

 by the growth of the operculum-like integument in the larval Salientia. 

 A prolongation of the tadpole stage would result in a retardation of the 

 growth of the fore-leg and an acceleration of that of the operculum. 

 The growth of the true skin of the inclosed region would be thus retarded 

 in the leg and atrophied in the wall of the chamber. (Plate 51, figs. 4-6.) 



The shoulder girdle appears separated from other parts of the skele- 

 ton, between the muscles. The coracoid and procoracoid form a loop, 

 directed downwards and inwards, far removed from that of tbe opposite 

 side, and present at this time an arciferous type in all forms of the order 

 Salientia. (See fig. 59.) 



Tbe characters of the cartilaginous skull of the larviB of the Salien- 

 tia are peculiar and very different from those of the adult. Tbe sus- 

 l)ensorium of the lower jaw is exceedingly elongated forwards, so that 

 for tbe purpose of securing a fixed point for the lower jaw (represented 

 now by Meckel's cartilage) it sends upwards a process near its anterior 

 extremity to the external angles of the cartilaginous ethmoid, forming an 

 articulation. It then descends again, and Meckel's cartilage articulates 

 freely with its extremity. There is a curved cartilage attached to the 

 extremity of each Meckel's cartilage (the t wo forming a half circle, open- 

 ing forwards), which form the support of tbe functional lower lip in the 

 larva. These are the lower labial or sympbyseal cartilages, and are rep- 

 resented in the adtilt by a pair of short bones of tbe same name (meu- 

 tomeckelians of Parker). Tbe premaxillary bones are in like manner 

 represented by two cartilages, which are loosely attached above to the 

 two corresponding processes or cornua of the trabecular cartilage, which 

 form the roof of the mitzzle in front of tbe ethmoid. (Plate 50, fig, 2.) 

 Tbe ceratobyal is a robust bone, which articulates with tbe quadrate 

 cartilage below the orbit, contracting in diameter as it extends down- 

 wards and forwards. In the process of growth its articulation with tbe 

 quadrate becomes more and more posterior, until it leaves that element 

 entirely, and comes in contact, and in some cases fuses, with the carti- 

 lage of the base of the skull in front of the stapes and near tbe inter- 

 stapedial. (Plate 50, fig. 3.) 



Besides the structures of the larval hyoid apparatus already de- 



