DEVELOPMEXT 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 uu ossified auditory capsules, right and left of the basisphenoid, there is a pair of 

 new investing bones ; these are the " basitemporals " (PI. LXVI. fig. 3, h.t) ; they are 

 uncinate shells of bone with a thin, toothed hinder margin, and they form a floor to 

 the cochlear pouch (chl). These were first found and described by me in the Chic^ 

 (see Trans. Zool. Soc. vol. iv. p. 280, and Phil. Trans. 1869, pis. 72-77, b.t) ; 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 " lingulse sphenoidales," and these 

 have tlieir largest development in Cavia cohaya. 



The parostoses of the mandible (PI. 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" 

 [cr.c), and the supraangular and angular {s.ag, ay) 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 (Pis. LXVI., LXA^IL). 



6th Stage. Embryo of Crocodilus palustris, taken August 8th, 5^ inches long. 



In this stage we find the jjeriotic hones begun, and indeed rapidly developing ; an 

 inner view of a vertical section (PI. 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 (b.o) now forms a large, rhomboidal plate, separated from the 

 exoccipitals (e.o) by a widish synchondrosis, a tract of cartilage which runs also across, 

 in front, between the basioccipital and basisphenoid (b.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, e.o) 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 {b.o, s.o). 



The hypoglossal nerve (xii) 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.o) 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 (ej)). 



2t2 



