DBVELOPJIENT OP THE SKULL IN THE UEODELES. 191 



trabeculae) with the auditory capsules, is largely ossified by continuous (probably 

 ^primarily) prootico-occipital centres {pr.o, e.o) ; these show division below but not above. 

 Below, even in the larger specimen (PI. XXXVIII. fig. 6), the middle third of the 

 capsule is cartilaginous ; this cartilage can be traced along inwards and forwards into 

 the clinoid band, across, and into the side wall, lengthwise, as far as midway between 

 the optic and trigeminal nerves (ii, v). 



Above, even in the young (PI. XXXIX. fig. 2), there is only one bone right and left 

 {pr.o, e.o) ; and this extends from the occipital condyles to the otic and ascending pro- 

 cesses (ot. p, a.p), in which it ends as a broad plate, looking forwards and a little outwards. 

 The square part which runs into the otic process, and the rounded growth of bone 

 which runs into the ascending process, are connected by a narrowish belt of bone which 

 forms the fore boundary of a pyriform cavity, partly covered in by a process from the 

 outer tract of bone ; this cavity, which runs into the auditory capsule, ends behind in a 

 sigmoid slit, which turns inwards and then a little outwards, following the outer margin 

 of the eminence caused by the anterior canal {a. s. c). It is bounded outside by the 

 horizontal canal (/*. s. c) in the same manner. This is the " aquseductus vestibuli (aj. v)," 

 and is to be seen again in Polyodon (see Bridge, Phil. Trans. 1878, pi. Ivi.); it is the 

 original auditory involution left unclosed in the capsule. 



The auditory capsule sends out a double horn on each side; these projections are 

 falcate, and have their convexity looking inwards and backwards, like the 'parietal 

 hones in the Lacertilia. 



These two pairs of projections, the outturned occipital condyles, and the parietal 

 bones have all the same contour, and are all parallel with one another ; the pedunculate 

 stapes {st) adds a fifth to these postero-lateral wings of the skull, the whole form of 

 which, from end to end, is that of a flat, subtriangular axis growing out m all directions 

 into spines and leafy growths. 



The stapes {st) is pyriform, and ends in a fiat notched lobe, which may be an 

 additional part that has coalesced with it, a pharyngohyal. 



The upper and lesser wing of the auditory capsule is the epiotic projection (ep) ; it is 

 bound down by the parietal horn and the sharp apex of the squamosal ( jj, sg) ; the 

 lower, larger, and outer horn is the " pterotic " projection, a part very large in fishes, 

 and well seen in some large American Frogs {e), Bana pipiens, Cystignathus ocellatus. 



Leaving out for the present the roots of the suspensorium {a.p,pd, ot.p), we see a 

 remarkable pair of postorbital projections [sp.o) growing in front of the prootic region 

 {pr.o), but not ossified by it ; they are between the ascending process {a.p) and the otic 

 process {ot.p), and may be called the "preauditory horns." These growths (as we 

 shall see in the larva of Triton cristatus, PI. XL.) are productions of the investmg 

 mass or hinder parachordals, and of the trabecular apices ; for these oblique laminee, the 

 hinder parachordals, wind round the front of the capsule, and appear in front of its 



