NECTURUS MACULATUS. 409 
forms with the condyle a right-angled triangle, the right angle facing the middle line and 
the hypothenuse forming the lateral side. The small interval formed between the oppos- 
ing right angles of the two exoccipitals is spanned by a ventral cartilaginous arch, the 
arcus basi-occipitalis, which, with the supra-occipital arch and the two bones under consid- 
eration, completes the enclosure of the foramen magnum. 
This basi-occipital cartilage is of greater extent in young and larval animals, and the 
ventral alae result from the gradually increasing ossification of its lateral ends. As this 
process continues towards the center, the inner angles of the bone nearly touch one 
another in old adults, while the cartilaginous arch is correspondingly diminished. 
The exoccipital comes into contact with four bones: (1) the parabasale, which is 
closely applied to the basi-occipital ala, (2) the opisthotic, which, in adults, touches both 
the body and the dorsal margin of the supra-occipital ala, (5) the parietal, a bit of the 
posterior margin of which becomes applied to a corresponding portion of the dorsal mar- 
gin of the supra-occipital ala, within its contact with the opisthotic, and (4) the atlas, 
which articulates with the condyles by a movable joint. 
The natural relations of an isolated exoccipital are best learned from the flat ventral 
surface. The condyle is posterior and the long straight edge is internal. 
4. quapRatuM. This is primarily a cartilaginous element, associated with the 
primordial skull, and representing the proximal (posterior) end of the palato-pterygo- 
quadrate arch, the functional upper jaw of the Selachians. In the Anura this arch is 
entire, but in Necturus it is represented by its two ends alone: the antorbital process, 
which represents its anterior, and the quadratum, its posterior portion. 
Functionally it serves as a “suspensorium,” or piece interposed between the skull 
and the mandible, and forming an articular surface for the latter. This joint occurs at 
its outer anterior angle, and that region of the quadrate becomes ossified, plainly to give 
strength to this very important joint. It thus happens that there is in the adult, both an 
osseous and a cartilaginous quadrate, the former being situated externally and the latter 
towards the median line. 
The irregular shape of the quadrate taken as a whole may best be seen by a reference 
to figures 10 and 11 (plate 64). The external osseous portion consists of an anterior 
articular process of very hard bone, fitted with an articular socket to receive the rounded 
cartilaginous knob (articulare) of the mandible, and a hollow trough-like posterior process 
fitted over the outer edge of the cartilage much as in the case of many of the dermal 
bones. The cartilaginous portion consists of a flattened plate which attaches by its broad- 
est side to the bony portion and tapers down to a narrow isthmus as it approaches the 
skull. Contact with the latter is formed by means of anterior and posterior extensions of 
the isthmus, which become applied to the outer side of the trabecula just in front of the 
