DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 15 



ON THE TYPICAL BATRACHIAN SKULL. 



In the following descriptions I shall take the skull of Rana temporaria as the 

 " norma " or pattern form ; it is the best on the whole ; one or two exceptional cha- 

 racters exist in it, viz. : the mark of the originally separate metapterygoid (a rare 

 character) ; the annulus tympanus is not perfect until old age ; and the stylo-hyal is a 

 long while fusing with the floor of the skull and ear-capsule, if indeed it ever becomes 

 fused ; but, with these exceptions, this may be taken as the highest kind of Batrachian 

 skull, and the best rule to measure tlie others by. 



ON THE EARLY STAGES OF THE BATRACHIAN SKULL. 



No known kind of Vertebrate shows so many and such instructive stages in its 

 development as the Batrachian. 



The skull of the newly-hatched einbiyo of the common Frog or Toad is strictly 

 comparable to that of a larval Lamprey {Ammocoeies), whilst that of the well-grown 

 Tadpole comes very close to the skull of the adult Petromyzon. 



But w^hilst the ordinary Batrachia have a very Petromyzine or suctorial larva, the 

 " Aglossa " in their early stages have a cranium and face very similar to what is seen 

 in the " Siluroid " Teleostean Fishes, and their skull suggests to the observer the most 

 probable form of endocranium likely to have existed in such Ganoids as Plerichthys 

 and Coccosteiis. 



Both these kinds of larval endocranium (chondrocranium) are figured in my 

 "Batrachian Skull," Part II. (Phil. Trans., 1876, Plates 54 and ^5, Biifo vulgaris; 

 Plates 56-58, Dactylethra ; and Plate 60, fig. 3, Pipa). 



My present business is with the suctorial type of larval Batrachian skull ; for full 

 details and figures of the wide-mouthed Siluriform type of skull I must refer the 

 reader to the paper just i-efei-red to. 



My most successful dissection of the earliest cartilaginous skull in these types was 

 that of Bufo vulgaris {op. cit., Plate 55, figs. 1, 2) ; the embryo was 4 lines (^rd of 

 an inch) in total length ; the next to this (Plate 55, fig. 3) was of a Tadpole of the 

 same species, 5 lines long. 



I shall make the first of tliese my First Stage, referring the reader to the plates in 

 the published paper. 



Of the skull in more advanced stages the pi-esent work will give many instances 

 and illustrations ; after describing the simple foundation, as seen in Bufo, I shall 

 describe a series of stages in various species of the genus liana. 



The larval skulls of other types will be described in their proper order, with the 

 adult condition of the skull in the same and other species. 



