522 



CHORD ATA. 



bone, surround the foramen magnum, the opening through 

 which the spinal cord passes to connect with the brain. These are 

 a pair of exoccipitals, right and left, a supraoccipital above and 

 a basioccipital below. The otica depend in their development 

 upon the extent of the otic region. In the fishes, where this part 

 is large, several bones may be present : epiotic, pterotic, sphenotic, 

 prootic, and often opisthotic. In the mammals, on the other hand, 

 these are fused to a single petrosal bone (figs. 466, 467) of small 

 size. 



Since the otic bones usually do not reach the middle line below, 

 the sphenoidalia rest direct upon the basioccipital behind and in 

 front upon a presphenoid bone, both unpaired but arising from 



Fig. 561.— Skull of Koat. (Frcim Clans.) Als^ alisphenoid: Bs, basisphenoid: C, occip- 

 ital condyle; EtU, mesethmoid, covering the ectetlinioid: Fo, optic foramen in 

 orbitosphenoid: Fi\ frontal; Inix. premaxillary; 7p, interparietal : Ju, .iugal 

 (malar); -La, lachrymal ; Mx, maxillary; Nn, nasal: (_th, basioccipital; 01. exoc- 

 cipital ; Ors, orbitosphenoid; Fa, parietal; Pa/, palatine: ft-, petrosal; P"*, 

 paramastoid process ; P.-j, presphenoid ; Pf, pterygoid : sf, frontal sinus in frontal 

 iDone ; ^pb, basisphenoid ; Sq, squamosal ; 3'//, tympanic; Vo, vomer. 



paired centres. With each is connected, right and left, a pair of 

 bones ; with the basisphenoid the alisiihenoids, with the presphenoid 

 the orbitosphenoids, just as the exoccipit;ils flank the basioccipital. 

 In the region of the nasal capsule there is an unpaired mesethmoid 

 with a pair of ectethmoids. Hence the cranium of primary bones 

 may bo described as a chain of four median basal bones, basioccipi- 

 tal, basisphenoid, presphenoid, and mesethmoid; right and left of 

 this a row of exoccijiital, alisphenoid, orbitosphenoid, and ecteth- 

 moid. The position of tho otic capsule results in the sum of the 

 otic bones, the petrosal, l)eing wedged in between the exoccipitals 



