DEVELOPMEXT OF THE SKULL IN THE CEOCODILIA. 285 



bj' the hind border; the articular part becomes an oblong sinuous condyle passing 

 across, and a little forward, externally (PI. LXV. figs. 5 & 8, 5). 



The quadrate bone (q) possesses the body of the cartilage, but not its outgrowths or 

 processes ; it is not jmeumatic at present. The free articulo-Meckelian rod or mandible 

 (PI. LXV. fig. 8, ar, mk) is nearly as long as the skull, has a saddle-shaped condyloid 

 facet [ar.c), a rounded angular process, a bony centre, the " articulare," and a long, terete, 

 Meckelian rod, which is confluent with its feUow in front (PI. LXVl. fig. 5, mk, h.mn). 



In the coronoid region there is a small, notched, squarish plate of cartilage, the 

 " coronoid cartilage " {cr.c) ; the rudiment of the continuous coronoid crest of the 

 mandible in Lejjidosteiis and Amia. Meckel's cartilage is thickest in the middle, and 

 is attenuated at each end. 



Another remarkable rudiment (or remnant) is seen in the upper part of the first 

 arch; this is an extrapterygoid facet (PI. LXV. fig. 8, and PI. LXVL figs. 1 & S,j)g.c') ; 

 this is a tongue-shaped tract lying along the outer edge of the pterygoid bone, where 

 it glides against the mandible ; it is a partial reappearance of the large ichthyic 

 pterygoid outgrowth. 



I shall describe the hyoid arch in both the Alligator and Crocodile in this stage, 

 which gives a stccpedio-hyoid bar, almost precisely the counterpart of that of Hatteria 

 (PI. LXVIII. figs. 15, 16, PI. LXIX. figs. 1-3; see also Huxley, Proc. Zool. Soc. 

 1869, p. 397, fig. 4). 



In the Alligator (PI. LXVIII. figs. 15, 16) the stapedial disk of the columella is very 

 large and oval ; it has its own ha si stapedial centre (st) on the inside of the disk. The 

 mediostapedial bone (m.st) is dilated on the outside of the disk, and forms a shortish 

 and slightly curved shaft, which reaches nearly to the distal dilatation of the columella ; 

 the segmental line seen at this part, in the earlier embryos, is gone. The bar itself is 

 continued upwards and forwards, but grows into a large fan-shaped crest, with a ribbed 

 free outer edge ; this is the extrastapedial process (e.st). From its thick (axial) back, 

 near its base, the suprastapedial process (s.sf) is given off; it is like a half-open fan, 

 and grows upwards, inwards, and backwards (see PI. LXV. fig. 8). Coalesced with 

 this, but with the line of junction still evident, we see the suprastapedial cartilage (s.s#), 

 an ear-shaped flap, twice as large as its stalk, and having its narrow lower end free. A 

 notch on the outer side of the broad, lower end of the extrastapedial receives the short, 

 curved epihyal {e.hy), and this is joined to a notch on the hinder side of the broad 

 upper end of the ceratohyal (chy), now membranous in its lower half, and therefore 

 quite free from the mandible below. These parts are all continuous (see also in the 

 irregular hyoid of another specimen, PI. LXVIII. fig. 16), and only show the old seams 

 of segmentation. 



The most perfectly Hatterian condition is seen in the hyoid arch of an embryo 

 Crocodile taken on July 27th (PI. LXIX. fig. 1). In this elegant stapedio-hyoid . 

 structure the stapedial base (st) has its own inner, osseous centre, and a stout bony 



