NECTURUS 19 



The chondrocranium (cartilaginous skull) can be studied by 

 taking the head of a fresh or alcoholic (not formalin) specimen, 

 removing the skin and dissolving the flesh, etc., by warm soap 

 solution, taking care that it does not get hot enough to injure 

 the cartilage. Then, with the scalpel the membrane bones, 

 enumerated above, may be readily removed, leaving but the 

 chondrocranium and the bones ossified in it. The nasal capsules 

 are usually so injured that they are not mentioned here. 



The chondrocranium consists behind of a pair of irregularly 

 oval hollow bodies, the otic capsules connected above with each 

 other by a narrow cartilage band, the synotic tectum, below by a 

 broader cartilage, the remains of a primitively much larger para- 

 chordal plate. On the medial side each otic capsule has a large 

 foramen for the passage of nerves and blood-vessels, while on the 

 inferolateral surface an opening, the fenestra ovalis, is occupied 

 by the stapes. The walls of the capsule are made up largely of 

 pro- and opisthotic bones, the extent of which can now be clearly 

 seen. 



From the anterior end of each otic capsule a slender cartilage 

 bar, the trabecula, extends to near the snout, where the trabeculae 

 of the two sides are connected by a median ethmoid plate, be- 

 yond which the trabeculae continue as the cornua trabecularium. 

 From each trabecula, near its anterior end, an antorbital process * 

 is directed outwards and slightly forwards, in front of the eye. 



The quadrate bone and cartilage are best considered together. 

 They extend outwards and forwards from the otic capsule to 

 the articular surface for the attachment of the jaw and to the 

 hinder end of the pterygoquadrate. From this as a centre they 

 give off two diverging processes, an otic, extending to the prootic 

 bone, and an epipterygoid process, reaching to the base of the 

 trabecula. 



This cartilaginous skull will also show well the stapes, a 

 rounded plate of bone, with a prominent columellar process, 

 fitting in the fenestra ovalis. 



The lower jaw consists of two halves loosely united in front, 

 and each half consisting of three bones arranged around a car- 

 tilage bar. These bones are a tooth-bearing dentary in front and 

 on the outer side; a plate-like, tooth-bearing splenial at about 



* Sometimes regarded as the anterior part of the pterygoquadrate arch f 

 but not proved to be such. 



