280 PEOF. W. K. PAEKER ON THE STETJCTUEE AND 



Below (PI. LXV. figs. 2, 3), the sigmoid cochlear rudiments [chl) grow towards the 

 notochord ; seen obliquely (fig. 3), there are two fenestrse, both oval in shape ; that 

 which is on the inside opens into the cochlea ("fenestra rotunda," f.r), and that 

 which is on the outside opens into the vestibule (/. ovalis, fs.o) ; the narrow tract 

 of cartilage between these passages is afterwards ossified by the "opisthotic bone." 



Before the anterior semicircular canal opens into the posterior it enlarges (fig. 3, 

 a.s.c, p.s.c), so as to have the appearance of one large arch with an " ampulla " at 

 each end. 



b. Visceral Arches. 



The inferior arches (PI. LXV. fig. 4, July 24th, and PI. LXVIII. figs. 12, 13, 14, July 

 20th) are now well developed, and bony shafts are forming in some of the rods on each 

 side. The anterior angular process of the quadrate (PI. LXV. fig. 4, q) is still larger, 

 and so is the orbital process, which is now at its fullest development — in this type \ it 

 shows a rudimentary ascending process and pterygoid cartilage {a.]), pg.c). The main 

 body of the quadrate is now elegantly scooped, from the solid part to the hind border, 

 so as to form a crescentic hollow, enlarging the tympanic space. The rounded lower 

 part above the condyle is enringed with an ectosteal tract, the quadrate bone; the 

 condyle is hemispherical at present. The joint was dislocated in the specimen figured, 

 and the columella appears too low down; the other figures (PI. LXVIII. figs. 12, 13) 

 of a younger embryo correct this. The saddle-shaped facet on the lower jaw is well 

 formed, and there is a considerable angular process. The Meckelian rod [mJc) gra- 

 dually lessens to half the thickness it has, proximally, and then turns outwards, and is 

 somewhat flattened at the end ; it is long and gently arcuate. Between the right and 

 left rods, in front, there is a wedge of cartilage interposed, the most distinct basal piece 

 (" basimandibular ") I have yet seen \ except in the Green Turtle (see Turtle's Skull, 

 op. cit. pi. 3. fig. 6). 



Noting a long facet of cartilage on the outside of the descending plate of the ptery- 

 goid bone (PI. LXV. fig. 8, pg.c'), I suspected that it would have its counterpart in 

 the mandible : it has (see PI. LXV. fig. 4, cr.c). This is the rudimentary " coronoid 

 cartilage," a large structure in the Lepidosteus and in Amia calva, but in them it forms 

 part of the mandibular rod. The " pterygoid cartilage " {pg.c') or facet on the descending 

 plate of the pterygoid bone is another equally instructive segment ; besides this separate 

 piece the lower horizontal fork of the quadrate {pg.c) shows a continuous remnant of 

 the large forth-growing pterygoid process of the UrodeJes and the Selachians. The 

 oval patch attached to the inner face of the coronoid bone (see PI. LXVI. fig. 2, 

 cr, cr.c) plays upon the patch above. The stapedio-hyoid chain is shown in situ 

 in PI. LXV. fig. 4 ; it is figured from a somewhat younger embryo, and detached 

 (PI. LXVIII. figs. 12, 13, 14); in that dissection I was able to make out a true 



' My friend Mr. Charles Stewart long ago pointed out to me this conjugation and dilatation of the distal 

 ends of the mandible in the embryo Crocodile. 



