SKELETON. 81 
the fronto-parietals. The otic capsule and quadrate are covered by a squamosal, 
and the roof of the mouth is formed by a large parasphenoid, in front of which are 
a pair of palatines. In advance of these last are a pair of large teeth resting directly 
on cartilage, their bases representing the greatly reduced vomers. The lower jaw 
has three bones on either side, a small dentary, a larger angulare, and an enormous 
splenial, which alone bears teeth. 
In Ceratodus there is a hyomandibular fused to the cranium behind the exit of 
the seventh nerve, but elsewhere there is only the hyoid. The operculum has one 
or two elements (operculare and interoperculum) the free edges of which bear 
Ras 4 7 
Fic. 81.—Skull of Lepidosiren, after Bridge. on, angulare; ap, antorbital process; ch, 
ceratohyal; cr, cranial rib; de, dermal ethmoid; dee, dermal ectethmoid; eo, exoccipital; fp, 
frontoparietal; #r, hyoidean ribs; mk, Meckel’s cartilage; na, first neural arch; nc, nasa 
capsule; msp, neural spine; pg, pterygoquadrate; sc, sagittal crest of frontoparietal: sp 
splenial; sg, squamosal; 1-10, nerve exits. 
cartilaginous rays, and the gill arches are five in Ceratodus, six in the other genera. 
A peculiar feature of Protopterus and Lepidosiren is the so-called head rib, a slender 
cartilage bone articulated with the chondrocranium below the occipital plate, and 
extending backward and downward across the shoulder girdle. 
In those extinct Dipnoi which are united with the recent genera to form the order 
Sirenoidea, the skull is much as in the existing forms, except for the more numer- 
ous bones. In the Arthrodira (formerly called placoderms) the cranium is hinged 
to a large plate which covers the anterior part of the trunk, and the skull is roofed 
with a few large plates, some of which may be homologized with those of the siren- 
oids, the others not being readily compared with the bones of other vertebrates. 
The suggestion has been made that the problematic fossil Paleospondylus resembles, 
in its skull, the larve of the dipnoans, the adults of which were common in the same 
seas. 
6 
