SKELETON. 



75 



fossa, and find the malleus and incus in the columella. For this they 

 have no evidence except comparisons with certain theriomorph reptiles. 

 The literature, which is extensive, should be consulted for details. 



The Skull in the Different Classes. 



CYCLOSTOMES have only the cartilage skull, and this can be homologized 

 only in part with that of other vertebrates; indeed the skulls of the two groups of 

 cyclostomes are not easily compared. The peculiarities are in part due to the 

 development of a suctorial mouth with its necessary framework. The chondro- 

 cranium of the Ammoccete stage of Petromyzon 

 is readily understood. Parachordals, otic capsules 

 and trabeculae (fig. 72) are normal, but a pair of 

 ventral horns are problematical. Their position 

 in front of and below the otic capsule renders 

 doubtful the interpretation of hyoid or quadrate 

 sometimes given them. 



The adult Petromyzon has a typical brain 

 trough, roofed by a slender synotic tectum and 

 fibrous tissue and closed in front by the unpaired 

 nasal capsule, bound to the rest by fibrous tissue. 

 The cranium is continued forward by a large plate 

 (mesethmoid?) lying dorsal to the mouth, this 

 part being roofed by two 'dorsal cartilages,' the 

 anterior articulating with the annular cartilage 

 supporting the mouth. A subocular bar extends 

 forward from each otic region and an elongate 

 lingual cartilage extends from the mouth back 

 to the gill region. Several other elements occur, 

 the names and positions of which may be seen from the figures. 



The myxinoid skull, the development of which is unknown, is readily inter- 

 preted so far as basilar plate, trabeculae and otic capsules are concerned. The large 

 nasal capsule is continued forward by a latticed framework for the naso-hypo- 

 physial canal and a bar (pterygoquadrate) joins the trabecula of either side and in 

 front is continued in a cornual cartilage. The lingual cartilage is enormous (is it 

 the lower jaw as has been suggested?), is divided into three segments and bears a 

 dental plate with teeth at its tip. There are cartilage axes to the tentacles around 

 the mouth. 



The branchial skeleton of the lampreys consists of a gill basket of continuous 

 cartilage with fenestrae for the gills and above and below them as well. It cannot 

 be homologized with the branchial skeleton of other vertebrates as it lies imme- 

 diately beneath the skin and is lateral to gill pouches and aortic arches. It is more 

 easily compared to the extrabranchials (p. 65) of the elasmobranchs. The 

 branchial apparatus of the myxinoids is reduced, consisting of two true gill arches, 

 in front of which is another arch, usually interpreted as a hyoid. 



He 



FIG. 72. Early chondro- 

 cranium of Ammocoete stage 

 of Petromyzon, after Schneider. 

 h, hyoid; nc, notochord; oc, otic 

 capsule; tr, trabeculae. 



