556 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
sac being larger than the right, and thrusting the suspensorium further forward 
(Plate 25. figs. 1, 2, au.q.). 
In this specimen the notochord showed more on the lower than on the upper side, 
and the sheath of bone was now unapparent, save perhaps as a slight groove on the 
parasphenoid ; in all the Urodeles this sheath is very evanescent. 
The exoccipitals ( eo .) had doubled in relative size, but I could not detect the prootic 
centres; this chondrocranium is therefore still the equivalent of that of the “Dipnoi.” 
A greater approach to the Selachian chondrocranium is evident in the growth inwards 
of more cartilage in the floor of the skull, making the fontanelle less and less. This 
is principally in front and at the sides, although, behind, there is more cartilage ; this, 
however, is a very temporary increment ; it soon wastes again there. The internasal 
plate has grown all round, and is very thick and solid at the middle ; the nasal roofs 
are now continuous with it and with the very broad cornua trabeculae ( c.tr .). Also we 
can see a greater distinctness between the fore end of the skull-wall and the precranial 
growths. Thickening of the cartilage, generally, has caused a greater obscuration of 
the canals of the ear-capsule as seen from without (fig. 1) ; below, the stapes has become 
free, and is now a large tongue of cartilage, with its broad end foremost and turned a 
little inwards. Behind, it does not yet accurately fit to the irregular fenestra ovalis. 
A bony (intrinsic) centre has appeared in the quadrate region (q.) ; it is spatulate, with 
the “ handle ” upwards. 
The large size of the left ear-capsule throws that suspensorium further forwards than 
the other ; its ascending process is confluent with the cranial wall ; the pedicle is still 
a knob, and the otic process a large blunt spur below, with a pedate process above. 
Now, with the superadded postpalatine cartilages, the chondropterygoid nearly reaches 
the antorbital {pg., a.o .) ; it is now, relatively, nearly as large as in the Selachian, 
whose mandibular suspensorium is a swinging quadrate, there being no pedicle or 
any ascending process, but only a huge chondropterygoid, growing anteromesially. 
This bar, in Siredon, bears no teeth, and the mouth is armed and surrounded by exo- 
skeletal toothed bones, that have no existence in the Selachian. 
The vomers ( v .) are now longer and more arcuate, and the little, toothed palatine bone 
is a small distance from the huge “ osteopterygoid.” 
Eighth Stage. Fully grown perennibranchiate Axolotls 8J inches long. 
This is a very important stage ; it is, indeed, the adult of this species, as a rule ; 
exceptional individuals undergo great metamorphic changes, but mostly this type grows 
to this large size and retains its gills. 
The higher kinds (Caducibranchs) are generally small ; and so is the Salamandrian 
form ( Amhlystoma ) which exceptional young Axolotls change into. 
As compared with the skull of an Amblystoma , this is low and larval; yet it has 
gained several new things since the last stage, and has become greatly modified in shape. 
This chondrosteous skull is a very exact intermedium between the chondrocranium of 
