1046 
SCANDINAVIAN FISHES. 
en up into two parts, an upper, which has medially 
grown together from the sides to form the framework 
of the palatine roof and an analogue to the quadrate 
bone (fig. 284, qu), and a lower (the Meckelian carti- 
lage), which forms the framework of the lower jaw (de 
+ M). At the age of three months these teeth dis- 
appear. Meanwhile the palato-quadrate cartilage has 
differentiated into two firmly coalescent, but distinguish- 
able parts, the posterior (mpt) answering to the meta- 
pterygoid of the Teleosts, the anterior corresponding 
principally to their pterygoid, proper and quadrate (qu), 
but also acquiring at its anterior margin special ossi- 
fications, homologous with the palatines (pi) and the ento- 
Fi°\ 285. Ossifications on the under surface of the skull in Acipen- 
ser sturio. J / 4 of the natural size, psp, parasphenoid; pa, its ascend- 
ing sphenoid process; vom, vomer; scr , vomerine scutes. 
pterygoids ( pt ). There further appears on each side of 
the anterior margin of the palatine arch a freer bone, 
the maxillary (mx), which extends from the anterior 
tip of the palatine disk (pterygoid bones) to its lateral 
extremity (the knob by means of which the quadrate 
bone articulates with the mandible), leaving between 
itself and the palatine disk a fissure, through which 
the levators of the lower jaw find a passage. On the 
outside of the above-mentioned juncture between the 
maxillary and the quadrate lies a triangular ossifica- 
tion (pop), most pointed above, which lias been inter- 
preted by Parker as representing a preoperculum; and 
on the outside of the Meckelian cartilage (M) is deve- 
loped the dental part (de) of the lower jaw. 
In the skull itself there appear ossifications the 
largest of which (psp) answers to the parasphenoid of 
the Teleosts, and extends from the nasal region not 
only under the skull, but also, divided into two lateral 
plates (fig. 285), under that part of the spinal column 
which has not been completely divided into separate 
vertebras, but in the form of a cartilaginous mass, con- 
tinuous below, composes a backward prolongation of 
the cranial cartilage. This parasphenoid is covered 
underneath, throughout the greater part of its length, 
by the hard mucous membrane alone; but between the 
nasal and orbital regions (in the ethmoidal region) it 
pierces the downward projection (fig. 284, B, the so- 
called basal angle, which may also be observed in the 
Sharks) of the chondrocranium. Within this it meets 
and wedges itself into the vomer (vom in figs. 284 
and 285) in front of it, which advances under the 
rostral cartilage (fig. 284, B), and is continued in its 
turn by several vomerine scutes (fig. 285, scr), evi- 
dently belonging to the skin. In the ethmoidal region 
(the lower anterior part of each orbit) the lateral eth- 
moids (fig. 284, etl) are developed; above the foramen 
of each optic nerve (in the arched roof of the orbit) 
appears an orbitosphenoid bone (fig. 284, obsp), behind 
this a smaller osseous disk, corresponding to the ali- 
sphenoid, and behind the orbit, round the orifice for 
the nervus trigeminus (tr), an osseous disk answering 
to the petrosal. But all these bones, according to Par- 
ker", are superficial growths external to the cartilage 
( parostoses ), foreshadowing the cartilage-bones (ectosto- 
ses and entostoses) present at the same points in the 
Teleosts and the higher vertebrates. Similar foresha- 
dowers appear in the form of osseous scutes (scale- 
growths) in the skin on the top of the head and outside 
the shoulder-girdle. At the middle of the occiput, but 
firmly united to the first dorsal scute, lies a trefoiled 
(posteriorly broad, with a narrower lobe projecting in 
front) osseous plate, evidently answering to the supra- 
occipital (here called the suprdoccipital scute, fig. 286, 
ocs) of the Teleosts. Further forward lie two larger 
scutes, usually the largest on the whole head, whose 
Philos. Trans. Roy. Soc. London, vol. 173 (1882), p. 175. 
