82 COMPARATIVE ANATOMY 



cranium : its upper element (pharyngohyal), articulating with 

 the skull to form the suspensorium, is then known as the hyo- 

 mandibular (Fig. 63), and from it (e.g., in Teleosts) a symplcdic 

 may be differentiated distally. 1 In the mid-ventral line there is a 

 basihyal connecting the arch of either side and embedded in the 

 tongue (entoglossal or glossohyaT). 



Certain smaller or larger skeletal parts of doubtful homology 

 form a kind of outwork to the skull anteriorly, and have been 

 described as prccranial or preoral elements. Under this category 

 are included the labial cartilages of Elasmobranchs (Fig. 63), and 

 similar structures amongst Teleostomes and in certain Anuran 

 larvse. 



Relations of the Chief Investing Bones to the Chondocranium. 



The primary relations of the investing bones to definite parts 

 of the cartilaginous skull are not in all cases sufficiently known, 

 but the following scheme, formulated by Gaupp, probably holds 

 good as regards the more important elements (cf. under Special 

 Part and Figs. 67-95). 



The parietals and frontals are primarily situated on the roof of 

 the skull in the auditory and orbitotemporal regions, beyond which 

 they may, however, extend. The squamosal, which in the Amniota 

 is developed on the outer wall of the auditory capsule, is also 

 present in Bony Ganoids and Teleosts, but in them loses the 

 character of an investing bone (cf. p. 83). 2 



The investing bones of the ethmoidal region are : the nasal, 

 supracthmoid of Teleostei, prefrontal of Amphibia and Sauropsida, 

 septomaxillavy (situated in the posterior region of the nasal 

 fenestra in Amphibia and Reptilia, and sometimes extending into 

 and beyond the nasal capsule), and the lacrymal of Mammalia ; it 

 is doubtful whether the last mentioned corresponds to the 

 similarly-named bone of Lizards and Crocodiles. The relations of 

 the prcmaxilla, maxilla, and vomer to the ethmoidal skeleton are 

 possibly of a secondary nature (cf. p. 83). The parasphenoid arises 

 in the mucous membrane beneath the cranial floor. 



On the lateral surface of the palatoquadrate of Amphibians is 

 an extensive bone, usually known as the squamosal, but called 

 the paraquadrate by Gaupp, who considers it to be homologous 

 with the quadratojugal of Reptiles. The vomer, palatine, and 

 pterygoid (including the ccto- and cnto-pterygoids of Teleosts) 

 probably arose as tooth-bearing investing bones on the palatine 

 region of the palatoquadrate, and are therefore traceable to the 

 teeth which in Elasmobranchs are present along the cartilaginous 



1 It is possible that the hyoid arch is the derivative of two arches which were 

 primarily separated by a cleft. 



' The primary relations of the postfrontal and jugal of the Amniota cannot 

 at present be stated. 



