NECTURUS MACULATUS. 409 



forms with the condyle a right-angled triangle, the right angle facing the middle line and 

 the hypothennse forming the lateral side. The small interval formed between the oppos- 

 ing right angles of the two exoccipitals is spanned by a ventral cartilagmous arch, the 

 arcus basi-occipitalis, which, with the supra-occipital arch and the two l>ones under consid- 

 eration, completes the enclosure of the foramen magnum. 



Tliis basi-occipital cartilage is of greater extent in young and larval annnals, 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 dimmished. 



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, (3) 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 po.sterior and the long straight edge is internal. 



4 QUADRATUM. This is primarily a cartilaginous element, associated with the 

 primordial skull, and representing the proximal (posterior) end of the palato-pterygo- 

 quacbate 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 mandibk, 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 lO^nd 11 (plate 04). The external osseous portion consists of an antenor 

 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 ot 

 the isthmus, which become applied to the outer side of the trabecula just in trout ot the 



