THE LOWER VERTEBRATA. 181 



cranium, and of jaws and visceral arches. The cranium 

 persists throughout life, in what closely resembles a trans- 

 'itory embryonic condition of the higher types. There is a 

 pair of olfactory capsules in front, a brain case proper, 

 and lateral otic (auditory) capsules containing the internal 

 ear. This should be compared with the tadpole's skull 

 (fig. 80). The upper jaw has a great bar of cartilage, the 

 palato-pterygoid, as its sole support ; a similar bar (Meckel's 

 cartilage) supports the lower jaw. In the frog these bars 

 of cartilage are connected directly to the otic capsule by a 

 quadrate cartilage, but this is only doubtfully represented 

 in the dogfish by a thin plate of cartilage just in front of 

 the spiracle. Instead, we find the jaws articulated with the 

 skull by a large cartilage behind the spiracle the hyoman- 

 dibular, the uppermost division of the hyoid arch. A skull 

 with this arrangement is termed hyostylic, as distinguished 

 from the autostylic skull of the frog. The rest of this 

 hyoidean arch on either side consists of a cerato-hyal, 

 joined ventrally to a median basi-hyal. 



Five branchial arches * follow, supporting the walls of the 

 pharynx between the gill-slits. An ideal branchial arch 

 would be a complete hoop of cartilage, encircling the 

 pharynx, and divided on each side into divisions, named 

 from the dorsal side downwards pharyngo-, epi-, cerato-, and 

 hypo-branchials, with a median basi-branchial in the floor of 

 the pharynx (cf. fig. 89, left side). But only in the first 

 two do the pharyngo-branchials of each side meet dorsally 

 to form a complete hoop ; and the two last pharyngo- 

 branchials are fused together on each side. The fifth arch 

 has no hypo-branchial. As to the basi-branchials, the first 

 one is fused up with the basi-hyal ; the other four are united 

 into a single plate, the basi-branchial. Obviously the 

 " hyoid plate " of the frog, and " body of the hyoid " of 

 the rabbit or dog represent basi-hyal plus basi-branchial. 

 The dog's " anterior cornu " is the main part of the hyoid 

 arch, and its " posterior cornu " is the first branchial arch. 

 (The frog's posterior cornu appears to be the fourth branchial 



* There is some confusion in the use of the term " arch " : it is 

 sometimes applied to the cartilages alone, at others to the whole side- 

 wall of the pharynx between two gill-slits. 



