OF THE SKULL IN THE UKODELOUS AMPHIBIA. 
595 
Their generic term may be “ pleural,” but that should not suggest the idea of a true 
rib or costal arch. 
Perchance these two species of arches are but secular specializations of one and the 
same type of ventral arch ; but that is a thing hidden from us now. 
I can only find two pairs of rudiments of visceral arches in front of the mouth — the 
eth mo-palatines, which are often separate elements, and the trabecular cornua (with the 
intervening “rostrum”); but these latter are exogenous processes from the paired basal 
plates growing into the frontal wall of the embryo. 
Description of Plates 21-29. 
Plate. 
Fig. 
Stage. 
Number of 
diameters 
magnified. 
21 
1 
1st | 
Unhatched embryo of Siredon, \ inch long. Side 
view of fore part 
j 26 
„ 
2 
55 
The same. Upper view 
26 
55 
3 
55 
The same. Under view 
26 
55 
4 
55 
The same in section (vertical) 
26 
55 
5 
2nd | 
Unhatched embryo of Siredon, ^ inch long. Side 
view of fore part 
} 20 
55 
6 
55 
The same. Upper view 
20 
55 
7 
r 
The same. Under view 
Hatched embryo of Axolotl, 5-| lines long. Trans- 
20 
} 
” 
8 
3rd < 
verse section (7th) of skull through middle of 
auditory region 
l 20 
55 
9 
„ 
A similar section (8th) further back 
20 
10 
55 
Another section (9th). Occipital region .... 
20 
22 
1 
2nd | 
Skull of unhatched embryo of Siredon, 4\ lines 
long. Upper view 
| 25 
55 
2 
55 
Visceral arches of same. Upper view .... 
25 
» 
3 
3rd | 
Skull of hatched embryo of Axolotl {Siredon), 
5 lines long. Side view 
} 20 
„ 
4 
4th 
Skull of young Axolotl, f- inch long. Upper view . 
15 
55 
5 
55 
The same. Lower view 
15 
55 
6 
5th 
Skull of young Axolotl, lj inch long. Upper view . 
10 
55 
7 
55 
The same. Lower view 
10 
