36 MR. W. K. PARKER ON THE STRUCTURE AND 



All the structures are now rapidly passing into their permanent foi-m, although 

 the individuals are, at this time, very small. 



The endocranium is here of the typical form ; a flat-bottomed " barge," with one 

 large anterior, and two small posterior membranous spaces in the " deck." 



The larger lateral expansions behind are connected with the lesser lateral expan- 

 sions in front, by an elegant bow of cartilage, the pterygo-palatine {pg., e.pa.) ; the 

 hinder expansions are caused by the impaction of the ear-masses into the sides of 

 the skull, and the expansions in front are due to the nasal growths ; from the open 

 spaces between, on each side, the eye-balls have been removed. 



The two pairs of ossifications in the endocranium, the prootics and the ex-occipitals 

 (2^>'-o., e.o.), are considerably larger than in the last stages ; the hinder pair have quite 

 encircled the twin passage for the 9th and 10th nerves ; a trace of the notochord (nc.) 

 still remains. 



There is no girdle-bone yet ; thus, with the exception of the " centres " just men- 

 tioned, the endocranium is a " chondrocranium." The form of the contents of the 

 ear-capsule is well seen through tlie semi-transparent cartilage, and the form and 

 extent of the originally separate nasal roofs, as distinguished from the pre-cerebral 

 region of the cranium, are well seen. Also below (fig. 7), the manner in which the 

 " intertrabecula " has filled in the space between the " cornua trabeculaj," and the 

 sub-division of the root of the " horns " into a pro-rliinal hook, and a subnasal outer 

 angle {p.rh., s.n.L), are clearly seen; also the divisions of the upper labials (fig. 6, 

 u.lKu.l-.), and their relation to the outer nostril (e.«.). 



The " ethmo-jsalatiue " bar (e.jia.) is lobate ; the pre-palatine is spiked, and the 

 post-palatine is a gently lessening bar, which witli the pre-palatine region in front 

 and pterygoid region {2^g.) behind, forms a most elegant subocular "bow." 



Behind, this bowed bar ends in three lobes ; the quadrate with its reniform condyle 

 (q-c.) running outwards, and downwards, and backwards; the "pedicle" {pel.) pedate, 

 with a flat inturned facet below ; and the " otic process " above, clamping the outer 

 and front end of the " tegmen tympani." 



For articulation wth the quadrate we see the cylindroidal condyle of the mandible 

 (fig. 8, ar.c); the under and inner face of the long arched rod is invested by the 

 "articulare"(a)'.) nearly to the fore end; the "dentary" {d.) runs along the distal half 

 on the outside, and it is grafting itself, near its lower end on the lower labial, which 

 is not quite confluent with the mandibular rod {ink.), and which is itself ossifying, to 

 become the " mento-Meckelian " bone {m.mL). 



The obliquely semi-oval stapes (figs. 7 and 10, st.) has now wedged in, between its 

 fore margin and the ear-capsule, the proxnnal end of the " columella " {epi-hyal element). 



This structure is not yet finished, and has a subdivision in it very rarely seen in 

 adults, but which is normal in certain fishes {e.g., the " Acipenserid;e "). 



These two parts are, morphologically, the proximal or " hyomandibular," and the distal, 



