1(58 Uli. W. K. PARKER ON THE STRUCTURE AND 



The tegnien tympani is shoAving itself, and outside the parachordal floor the 

 obhque fenestra ovaUs is vearly filled with the oval stapes (st.), now well chondrified. 

 The unfilled fore end of the fenestra ovalis is waiting until after most of the meta- 

 morphosis is done to receive the columella, which will articulate with the newer 

 parachordal floor at its outer edge. Cartilage has sjjread from the tegmen tympani 

 into the angular space between the capsule and the otic process of the suspensorium 

 (ot.j^.), just as cartilage spreads from the inner edge of the capsule into the mem- 

 branous superoccipital tract — for cartilaye, like hone, in many instances, does not heed 

 morphological landmarks. The little cordiate cartilage (fig. 7, sp.c.) which is develop- 

 ing in the opercular membrane, over the 1st cleft, becomes free from the ear-capsule, 

 coalesces with the otic process of the suspensorium, becomes detached again from that 

 part, and then develops into the annulus tympanicus, and is permanently kept as the 

 cartilaginous " operculum " of the cleft to which it belongs ; the specialisation of both 

 cleft and operculum failing to disturb the original morphological relation of the parts. 

 In like manner the modified hyo-mandibular or columella, will be wedged in between 

 the auditory capsule and the basal plate, exactly as in its simple and generalised 

 counterpart in the Selachians. 



The suspensorial arch has the normal form ; the pedicle (fig. 7, inside sp.c.) is 

 narrow, the otic process (fig. 6, ot.p.) is a thick crest, ascending from the main bar as it 

 suddenly turns inwards to become the pedicle. The quadi'ate condyle is a small 

 trochlea, and looks inwards as well as forwards, and the terminal or quadrate I'egion 

 [q.) is altogether oblique ; it sends inwards an angular pre-palatine spur which is 

 attached to the cornu trabeculse in front of the narial passage {i.n.) by a ligament. 



The orbitar process (fig. 6, or. p.) is large and rounded, with raised edges ; the 

 hyoidean condyle beneath it has the usual reniform outline, and is scooped. The 

 wings of the ethmoid are very rudmientary at present, and I find no post-palatme 

 rudiment outside them. The mandibles (fig. 7, ink.) are short, thick, ulniform rods, 

 with the usual hooked angular process ; they are articulated to the lower labials by a 

 wide surface ; these latter are each half a thick crescent with a raised lip. The 

 temporary upper labials (»./.) are large, thin, crescentic flaps fitted on to the end of 

 the cornua trabecule. The hyoid bar (fig. 7, c.lvj.) has the usual form ; it has a large 

 condyle and an unciform stylo-hyal free end ; it has a narrow waist, obhquely ridged, 

 externally, for muscular attachment, and then expands largely, most on its hinder 

 margin, to contract again where it is joined to its fellow, ventrally, by the square, soft- 

 celled, basi-hyal. The branchial arches were not figured in this instance, they were 

 quite normal. 



The parasphenoid (fig. 7, pa.s.) formed a floor, very exactly, to the newer cai-tilage 

 of the intertrabecular space ; it had, although it was very thin and yoimg, developed 

 the rudiment of its lateral and posterior angles; the last was directly under the 

 (endoskeletal) bony sheath of the notochord. 



