196 PHYLUM CHORDATA 



are the two exoccipital bones. Each exoccipital bears on its hinder 

 surface a convex articular projection, the occipital condyle, by 

 which the skull articulates with the atlas. At the side of each ex- 

 occipital will be seen a portion of the primitive cartilage. 



The pair of long, flat bones which form the roof of the cranium 

 and lie directly in front of the occipitals are the frontoparietals. 

 Their anterior ends overlap a bony ring which encircles the for- 

 ward end of the brain case ; this is the ethmoid. 



The ventral portion of the cranium is formed by two bones, 

 the ethmoid, just mentioned, and the parasphenoid. The latter 

 is a large T-shaped bone which covers the entire ventral portion 

 of the cranium and overlaps the ethmoid ; its lateral portions 

 extend to the auditory capsules. At the sides of it, back of the 

 ethmoid, will be seen the primitive cartilage. The large foramen 

 of the optic nerve will be seen in the side of the cranium between 

 the frontoparietal and the parasphenoid. 



The Special-Sense Capsules. These are the auditory capsules 

 at the hinder end of the skull and the nasal capsules at its for- 

 ward end. The optic capsules do not ossify. 



The auditory capsules are fused with the sides of the cranium 

 proper and consist largely of cartilage. On the ventral side of the 

 skull the lateral projections of the parasphenoid bone cover them. 

 On the dorsal and anterior sides a bone called the prootic is pres- 

 ent ; it will be seen abutting the hinder part of the frontoparietal 

 bone. Between the prootic and parasphenoid bones is the large 

 foramen of the trigeminal nerve. 



Ventral to the prootic on the side of the skull is a depression ; 

 at the bottom of this is a large hole called the fenestra ovalis, 

 which looks inside the auditory capsule. The depression is the 

 tympanic cavity, — the middle ear of higher vertebrates, — which 

 in the fresh skull is covered laterally by the tympanic membrane, 

 or ear drum. The fenestra ovalis is joined with the tympanic 

 membrane by a small bony and cartilaginous rod called the col- 

 umella. This small structure, like the ossicles of the mammalian 

 ear, conveys the sound waves from the tympanic membrane to 

 the inner ear. Skulls from which the tympanic membrane has 

 been removed have often lost it. 



