DEVELOPMENT OF THE SKULL IN THE CROCODILIA. 289 
supero-externally, and scooped fore and aft; against this ridge lies the crescentic carti- 
laginous facet already spoken of; the direction of both is with the convex edge looking 
forwards and downwards. A bone, with the outline of an hourglass, fits by its top to 
the inside of the maxillo-jugal suture, and by its base lies under the fore part of the 
pterygoid wing, obliquely; this is the ‘‘os transversum” or “ transpalatine” (t.pa), 
with the maxillary and palatine it forms a large oval “ palatine fenestra.” 
All these bones were to be seen in the third stage; but between the pterygoid and 
the unossified auditory capsules, right and left of the basisphenoid, there is a pair of 
new investing bones; these are the “ basitemporals” (Pl. LXVI. fig. 3, 0.¢); they are 
uncinate shells of bone with a thin, toothed hinder margin, and they form a floor to 
the cochlear pouch (ch/). These were first found and described by me in the Chick 
(see Trans. Zool. Soc. vol. iv. p. 280, and Phil. Trans. 1869, pls. 72-77, 6.¢); I have not 
been able to find these “ parostoses” distinct in any other kind of Reptile; in the 
Mammalia they are manifestly represented by the “ lingulz sphenoidales,’ and these 
have their largest development in Cavia cobaya. 
The parostoses of the mandible (Pl. LXVI. figs. 1, 2) are now well developed ; the 
dentary (d) is by far the largest, the splenial (sp) is a very long splint, the coronoid 
(cr) is a small angular patch applied to the inner face of the “coronoid cartilage” 
(er.c), and the supraangular and angular (s.ag,aq) are styliform, and send their long 
sharp ends forward, over and under the mandibular fenestra (mn.f), which is large and 
oval; these mandibular splints are also figured in the sections (Pls. LXVI., LX VILI.). 
6th Stage. Embryo of Crocodilus palustris, taken August 8th, 52 inches long. 
In this stage we find the periotic bones begun, and indeed rapidly developing; an 
inner view of a vertical section (Pl. LXIX. fig. 6) shows these three bones, and also the 
alisphenoid and supraoccipital ; the bones that had begun in the last stage have grown 
very much. 
The basioccipital bone (0.0) now forms a large, rhomboidal plate, separated from the 
exoccipitals (¢e.0) by a widish synchondrosis, a tract of cartilage which runs also across, 
in front, between the basioccipital and basisphenoid (4.s). At that part the basis cranii 
is pneumatic, and the median part of the tympanic Eustachian labyrinth is seen there. 
A remnant of the notochord still exists in the basioccipital; it dimples the large 
transverse condyle (oc.c). The exoccipitals (figs. 5, 6, ¢.0) run to the top of the foramen 
magnum, grow well forward towards the auditory capsule, are separated by a widish 
tract below from the basal, and also by a wide tract above from the upper bone of the 
arch (4.0, 8.0). 
The hypoglossal nerve (x11) behind, and a vein in front of it, pierce the base of the 
exoccipital; it is notched for the vagus nerve (x). The supraoccipital (s.0) is a rhom- 
boidal plate formed in the hind roof-cartilage ; it is rather thick already, ready to become 
pneumatic; it articulates at its antero-inferior edge with the epiotic (ep). 
2x 2 
