6 GUIDES FOR VERTEBRATE DISSECTION 



and the rostrum is a large anterior fontanelle. (In some Elasmo- 

 branchs there is also a posterior fontanelle.) The roof of the 

 nasal capsule is thin and is usually torn. On the roof of the 

 otic capsule are two ridges running from near the endolymph 

 depression, one forward and outwards, the other obliquely 

 backward to the posterior angle of the skull. These indicate 

 respectively the position of the anterior and posterior semi- 

 circular canals of the inner ear. 



Draw the cranium from above, natural size. 



In the lateral view see that the orbit is bounded in front 

 and behind by strong vertical ridges continuous with the ant- 

 and postorbital processes. Make out also the regions of the 

 nasal and otic capsules. In the orbit recognize foramina for 

 the passage of nerves as follows: Near the ventral middle the 

 largest, optic foramen, for the optic nerve; in the roof of the 

 orbit the ophthalmic foramen already seen; at the lower posterior 

 angle a large foramen lacerum anterior for the fifth, sixth, and 

 part of the seventh nerves; directly in front of the last an 

 oculomotor foramen for the third nerve; and about half way 

 between the lacerum and the ophthalmic a small trochlear 

 foramen for the fourth nerve. A little ventral and in front of 

 the lacerum is the opening of the transbasal canal. At the 

 antero-ventral angle of the otic capsule is a large facial foramen 

 for the main trunk of the seventh nerve. In the lateral wall 

 of the otic capsule is a longitudinal ridge indicating the position 

 of the external semicircular canal of the ear. 



Draw a side view of the cranium, natural size. 



In the ventral view cf the cranium note the strong keel 

 produced by the median bar of the rostrum. In the nasal 

 capsule the narial opening is seen, and in the deeper portion of 

 the capsule the large opening by which the olfactory nerve enters. 

 Note also the narrow region between the orbits bounded behind 

 by a pair of basilar processes each bearing on its lateral surface a 

 palatobasal articular surface against which the palatal process 

 of the upper jaw plays. A little behind this basal process is 

 a median foramen for the passage of the united internal carotid 

 arteries. Note also the articular surface on the strong postero- 

 lateral angle of the ventral surface of the otic capsule with which 

 the dorsal end of the hyomandibular articulates. 



Draw a ventral view of the cranium, natural size. 



