DEVELOPMEXT OF THE SKULL IN THE CEOCODILIA. 291 



the figure the hinder part of the soft palate is cut away so as to expose the " posterior 

 nares" (i.n), which open into a circular recess. Behind them there is a smaller 

 notched fold of membrane, and over the notch a round median passage ; this is the 

 middle Eustachian passage {m.eu), it opens into the basis cranii. 



At a short distance outside this there is, on each side, a curved valvular opening ; the 

 broader end of each is behind, and these ends approximate ; the convex edge of each is 

 postero-lateral ; these paired openings are the lateral Eustachian openings {Leu), and 

 run directly into the main tympanic cavity. 



7th Stage. Isearly ripe, and ripe embryos o/" Crocodilus palustris, taken from Septemler 



ith to September 8th, total length 10 to 10| inches ; and ripe embryos o/ Crocodilus 



■?, sp. 



a. Endocranium. 



The general form of the parts of the endocranium alters but little daring the last 

 month — August 8th to September 8 th ; its changes are mainly increase of size and 

 consolidation of the various regions, especially the bony centres. The two paired and 

 the two unpaired bony tracts of the occipital arch (PL LXIX. figs. 7, 8, e.o,s.o,b.o) 

 now form a strong ring of bone, with very limited synchondroses dividing its elements. 

 The basioccipital (b.o) does not reach the basisphenoid (b.s), nor fill all the cartilage 

 belonging to it behind ; thus the condyle (oc.c) is a solid mass of cartilage, and not a 

 mere articular plastering left on the bone. The form of the bone is roughly pentagonal 

 (PL LXX. figs. 1 & 3, b.o), and it is separated from the other bones by narrow tracts 

 of cartilage. It is somewhat grooved in the middle, below, and mamillate right and 

 left, and its gradually narrowing hinder part is imbedded in the reniform condyle, 

 whose " hilus " is filled with the remnant of the notochord (w.c). A common round 

 opening leads into both this bone and the basisphenoid, for one pneumatic cavity 

 occupies both ; the opening is the " middle Eustachian passage " (m.eu). In the front 

 of each lateral angle there is another, lesser passage, which is the posterior opening of 

 the " lateral Eustachian tube " or passage (l.eu). This opens into a lateral chink behind 

 the basitemporal plate (b.t) (PL LXX. figs. 3, 4, & 8, l.eu), and this chink leads also 

 into the excavated basisphenoid, the anterior opening of the " lateral Eustachian tube." 

 The large, winged exoccipitals (e.o) and their syncliondrosial cartilages complete the 

 ring over the foramen magnum (PL LXIX. fig. 11, and PL LXX. figs. 1 & 3,/m). 

 They are large, irregular, multangular shells of bone — shells both in their general 

 convexo-concave form, and also because they are hollow or pneumatic. They are 

 riddled with holes on their lower surface, but each hole has its meaning and function. 



The hindermost of these, nearest the condyle, is for the hypoglossal nerve (xii), then 

 there is a small passage for a vein, and outside in front of that a hollow with two 

 passages, these are for the vagus and glosso-pharyngeal nerve (x, ix). At the antero- 

 internal angle there is a notable round hole ; this leads to a more or less perfect bony 



