DEVELOPMENT OF TTIE SKULL IX THE BATRACHIA. 255 



and each of these gives off four nidiinentary true, or iutra-branchial, cartilages — small 

 " cerato-branchials '' — with, as is the case in the hyoid, at present, no upper element. 



This inferior, arrested, wii?'a-branchial framework is covered with a " lower velum," 

 and the projection of the four pairs of rudimentary cerato-branchials makes it have a 

 crenate margin (Plate 1, fig. 4) ; it is the broad rudiment of the membrane which 

 runs under the pharynx in the Lamprey at the entrance of the "branchial canal;" 

 the copious gi-owths of gill- tufts on each side, divided by the three clefts, lie under it, 

 partially covered. 



All these lar\al structures may exist in the chondrocranlum before the Petromyzine 

 skidl is modified either by outer or inner bones, and the skull of the larva explains, 

 and is explained by, the skull of Petromyzon and its congeners. 



To the Morphologist there is, here, a pause ; but, in fixct, in the growth of the 

 Tadpole's skull, there is none ; changes begin soon, and the work of transformation 

 then goes on steadily. 



B. h. — Perfect chondrocranium — hefore the formation of hony centres — in 



the " Aglossa." 



The chondrocranium of the larval Dactylethra (Phil. Trans., 1876, Plate 56) is 

 extremely flat and outspread ; it has all the essential parts of a Batrachian larval skull, 

 nevertheless. Its form is almost triangular, with the base in front ; and although the 

 condyles of the quadi-ate are carried forwards almost as far as the trabecular horns, they 

 are a great distance apart. Even behind, the skull is greatly widened by lateral out- 

 growths of cartilage, a wide " tegmen tympani " already growing from each auditory 

 capsule ; and from this, and confluent with it, a very large ear-shaped " spiracular " 

 cartilage, which spreads wider than, and almost touches the base of, the small orbitar 

 process, and the hinge for the cerato-hyal. The suspensoria, also, are -wide bands 

 running, without an outbend, forwards and gently outwards, to the front of the orbitar 

 process. They then bend inwards and are confluent by a very wide band with the 

 trabeculae, each band being pterygo-palatine and pre-palatine all in one plate. As in 

 Skates, the trabeculae are enormous ; they are separated from the suspensoria by a very 

 narrow subocular fenestra, and rise as convexo-concave shells, converging at the hinder 

 third of the interorbital region to finish the cranial cavity. As in Skates, however, 

 there is a long hollow from it into the nasal trough, which soon widens again. The 

 comua trabeculse are .stretched out, as wide arms, in front, and their free end is 

 confluent with the pre-palatine spur. They are narrowish bands, and curve round 

 the front of nasal passages which are near the frontal margin, but very wide apart. 

 Their fore margin shows no notch, for they are completely fu.sed together by a large, 

 but indistinct, iuter-trabecular tract. The continuous cartilaginous labial, in front of 

 the coniua, is partly confluent with them, and runs wider out ; it is the same as the 

 "anterior dorsal cartilage" of the Lamprey; the "angular cartilage " of tluit Fish is 



