DEVELOPMENT OF THE SKULL IN THE BATRACHIA. '5 



condyle looks more diix'ctly forwards, and not so much inwards ; the orbitar process 

 is more elongated. 



The lower arches and labials (Plate 11, figs. 3-5, mk., LI., c.lnj., h.hij., h.hr., li.hr., 

 c.br., ex.hr.) are but little changed. 



17 (continued).— (C) Tadpole of P.'ietidis paracloxa, with all the legs large and free ; 

 tail, 5 inches long ; its greatest width li inch. 



The chondrocranium (Plate 11, figs. 6, 7 ; and Plate 12, fig. 1), although relatively 

 wider on account of the throwing out of the suspensorial bands, is only half the length 

 it had, even in the second stage (B), and a third the length of that of the first (A). 



This temporary transitional form is very extraordinary ; I have caught no stage 

 quite like it in any other species. 



The third Tadjjole of Rana clamata (Plate 4, figs. 5, G) comes nearest to it. The 

 ethmo-nasal region, which was one-half the length of the skull in the last stage, is 

 now less than one-third ; thus the orbital and auditory regions are now very large, 

 relatively, and with them the cranial cavity is equally mcreased in size (relatively). 



The occipital condyles (Plate 11, figs. 6, 7, oc.c.) are now wider apai-t ; but the ossifi- 

 cation here is but httle increased in extent. The bony substance has united over 

 the occipital roof, and a definite tract of cartilage lies in front of the bone. There 

 are no " secondary fontanelles," and only a wedge-shaped tract of the main fontanelle 

 (fo.) is uncovered by bone (fig. 6). 



The notochord (nc.) is still present as a fine thread, and the basioccipital synchon- 

 drosis is unaltered ; moreover, the floor of the vestibule below, and the outer part of 

 the capsule with the tegmen tympani, continue miossified. 



The semi-circular canals stand out strongly, and along the ridge of the posterior 

 canal and part of the anterior {})r.o., ait., ep.), the bone has undergone dehiscence, 

 exposing the cartilage within, and pai'tly separating the prootic from the ex -occipital 

 on each side (fig. 6, pr.o., e.o.). 



In hke manner, over the skull, the large frontals are partly severed from the small 

 parietals (/, p.) ; the parasphenoid {pa.s.), below, has become smaller (actually), and 

 more elegantly formed. 



The unossified auditory floor (fig. 7) is now a more transverse, sub-oval mass, and 

 the projecting, descending lip of the floor is very small (Plate 11, figs. 7 and 8 ; and 

 Plate 12, fig. 1, f.tij.). The upper part of the auditory capsule i)rojects beyond the 

 vestibular pouch, very considerably. 



The cranial cavity lessens in width gently to the fore end ; the '• tegmen " is of 

 moderate extent over it, and part of it passes uito the nasal region, as the roof of 

 the ethmoid. 



The optic nerve (Plate 11, fig. 7, II. ; and Plate 12, fig. 1, II.) passes out of the 

 binder part of a large fenestra ; the roof- bones have a down-turned orbital edge. 



I, 2 



