480 STRUCTURE OF VERTEBRATA. 
SUMMARY OF THE DEVELOPMENT OF THE SKULL 
Onicin. REsuLTs. 
ELEMENTS. 
I. Parachordals 
and _ trabeculze, 
aided in some, 
cases by the end 
of the notochord. 
Their precise 
relations, ¢.g.to 
the notochord, 
are unknown. 
Occipital region, with four bones—basi-occi- 
ital, two ex-occipitals, and a supra-occipital 
lin part). The basi-occipital is distinct only in 
Reptiles, Birds, and Mammals. 
Sphenoidal and ethmoidal region, with basi- 
sphenoid and pre-sphenoid (present only in 
Reptiles, Birds, and Mammals), paired ali- 
sphenoids and orbitosphenoids, the inter-orbital 
septum, the lateral or ectoethmoids, the inter- 
nasal septum. 
II. Sensecapsules. 
(a) Nasal. 
(4) Auditory. 
From cartilage 
surrounding the 
ectodermic pits 
which form the 
foundation of 
nose and ear. 
, 
(a) Unite with ethmoidal region. 
(4) May give origin to five bones—pro-, 
sphen-, pter-, epi-, and opisth- otics, or to the 
single periotic of Mammals, 
III. Arches, 
(2) Mandibular. 
(4) Hyoid arch. 
These arches, 
like those which 
follow them, are 
supports of the 
pharynx, lying 
between primit- 
ive or persistent 
gill-slits. 
(2) Upper part = palato-pterygo-quadrate 
cartilage of Elasmobranchs, palatine, pterygoid, 
and quadrate bones in the higher Vertebrates, 
but in Mammals the quadrate is believed by 
many to become the incus of the middle ear. 
Lower part = Meckel’s cartilage—the basis of 
the lower jaw in all animals ; the part next the 
quadrate becomes the articular bone, which in 
Mammals is believed by many to become the 
malleus of the middle ear, 
(4) Upper part or hyo-mandibular=the ‘‘sus- 
pensorium”’ cartilage of Elasmobranchs, the 
hyo-mandibular and symplectic of -Teleosteans, 
the columella auris of Amphibians, Reptiles, 
and Birds, the stapes of the Mammal’s ear. 
Lower part=the hyoid proper (cartilage or 
bone). 
IV. Investing 
membrane bones, 
(az) From the 
roof of the skull. 
(4) On the floor 
of the skull, z.e. 
from the roof of 
the mouth. 
(c) On the sides 
of the skull, 
(d) On upper 
jaw 
(e) On lower jaw. 
Originally of 
the nature of 
external bony 
plates, tooth 
structures, and 
the like, 
(2) Parietals, frontals, nasals, ete. 
(4) Vomer, parasphenoid, ete. 
(c) Lachrymal, squamosal, orbitals, ete. 
(d) Premaxilla, maxilla, jugal, and quadrato- 
jugal (in part). 
(e) Dentary, splenial, angular, supra-angular, 
coronoid. Some recognise also a gonial, often 
fusing with the articular. 
