14 LABORATORY DIRECTIONS 



the cranium and is held to it merely by ligaments. Secure a 

 separate cranium for detailed study. Shark crania are fragile 

 and expensive. They must be handled with great care and must 

 be kept in their containers when not in use. Do not allow them 

 to become dry. 



Observe that the whole cranium forms one continuous 

 cartilaginous mass. This is known as a chondrocranium. 

 Although the cartilage is sometimes strengthened by impregna- 

 tion of lime, there is no real bone formation in any part of the 

 shark skeleton. 



Find the following outstanding features: 



1. Orbits (eye sockets). — The lateral projections in front of 

 the orbits are the anterior orbital processes; those behind the 

 orbits are the posterior orbital processes. Between these two 

 processes and forming a shelf above each orbit is the supra- 

 orbital crest. 



2. Rostrum. — This is the scoop-like projection at the 

 anterior end of the cranium, opening dorsally. The open cavity 

 of the rostrum is the anterior fontanelle, which in life is filled 

 with a gelatinous substance. Normally the rostrum is braced 

 by rostal bars, extending from its tip to the anterior orbital 

 processes, but in prepared crania these bars are usually missing. 

 On the ventral side of the rostrum is a keel. 



3. Foramen Magnum, a large opening at the posterior end 

 of the cranium for the exit of the spinal cord. 



4. Occipital Condyles, small projections, one on either 

 side of the foramen magnum, for articulation with the first 

 vertebra. 



5. Endolymphatic Fossa. — This fossa is the large depres- 

 sion on the dorsal surface anterior to the foramen magnum. 

 Within it are two pairs of openings: the anterior smaller ones 

 are the openings of the endolymphatic ducts; the posterior ones, 

 the openings of the perilymphatic ducts. These ducts connect 

 with the internal ears, which lie embedded in the cartilage of the 

 otic region. 



6. Otic Region (ear region). — Extending anteriorly on each 

 side of the endolymphatic fossa are ridges which indicate the 

 location of the anterior semicircular ca^ials. Similar ridges 

 extending posteriorly mark the position of the posterior semi- 



