SKELETON 



107 



usually intervening. In the 'euornithes' the articulation occurs. The latter 

 are subdivided into the desmognathous forms where the vomer is small or want- 

 ing, and the maxillopalatines meet in the middle line; the schizognathous in 

 which the maxillopalatines do not meet the vomer or each other; the aegithogna- 

 thous, like the last except that the vomer is broad and truncate; and the saurog- 

 nathous with delicate, rod-hke vomers and maxillopalatines scarcely extending 

 inward from the maxillaries. 



The chondrocranium of the MAMMALS has several peculiarities (fig. 112). 

 There are four occipital vertebrae, the last only with a complete vertebral char- 

 acter, all eventually fusing with the synotic tectum. The dorsal part of the otic 

 capsule chondrifies first, owing to the late development of the cochlear part of 

 the ear in the lower half; and the capsules themselves have their axes inclined, 

 so that the exit of the seventh nerve is on the anterior rather than on the lateral 



Fig. 113. — Diagram of the bones of the mammalian skull, altered from Flower, 

 tilage bones dotted, membrane bones lined; 2-12, nerve exits. 



Car- 



face. The trabecule soon join the basal plate, and from their sellar part an 

 alary process is given off on either side which extends upward to join an alisphe- 

 noid (ala temporalis) which chondrifies separately, but soon joins the otic cap- 

 sule above, leaving between them the foramen ovale for the third branch of the 

 fifth nerve, the other branches passing forward over the ala and then between it 

 and the orbitosphenoid (ala orbitalls) through the sphenoidal fissure (foramen 

 lacerum anterior). The ala orbitalis joins the trabecula by two processes, bar 

 and processes sometimes forming a reduced interorbital septum. Later a 

 marginal band (taenia marginalis) extends back from the orbitosphenoid to a 

 cartilage plate developed on the otic capsule. The ethmoid parts are compli- 

 cated, consisting of the two nasal capsules, the septum between them, and, on 

 the inside, coiled turbinal cartilages to support the olfactory membrane. 



Some of the visceral arches have been mentioned in speaking of the ear bones 

 (p. 80). The pterygoid cartilage is apparently lacking, and there is nothing that 

 can be interpreted as a quadrate except the incus. Meckel's cartilage extends 

 forward from the incus to the tip of the jaw. In the procartilage stage the 



