xii PHYLUM CHORDATA 411 



so that the only free parts are the lower jaw and a 

 small plate, the hyoid apparatus, partly bony and partly 

 cartilaginous, which supports the tongue, and is the sole 

 representative of the entire visceral or gill-bearing skeleton 

 of the fishes. 



A cartilaginous cranium comparable with that of the dog- 

 fish, but very thin and delicate, forms the foundation of the 

 skull of the frog ; but superadded to this are a number of 

 cartilage bones or bones which replace portions of the 

 cartilage, and membrane bones or bones which are formed 

 in membrane, independently of the cartilage. There are 

 five cartilage bones, the paired exoccipitals and pro-otics, and 

 the median sphenethmoid. The exoccipitals (EX. OC) lie 

 in the posterior or occipital region of the skull and bound the 

 large opening or foramen magnum (for. mag) at the posterior 

 end of the skull through which the spinal cord, contained in 

 the neural canal enclosed by the neural arches of the vertebrae, 

 becomes continuous with the brain, contained in the cavity 

 of the cranium. Below the foramen magnum are a pair of 

 oval projections, the occipital condyles (oc. en), for articu- 

 lation with the first vertebra of the spinal column. The 

 second pair of cartilage bones, the pro-otics (PR. OT), are 

 ossifications in the roof of the auditory capsule, situated 

 just in front of the exoccipitals, with which they become 

 firmly united. The sphenethmoid (SP. ETH) is a peculiar 

 ossification of somewhat complex form, which lies partly in 

 the wall of the anterior portion of the cranial cavity, partly in 

 the wall of the posterior portions of the nasal cavities or 

 olfactory sacs. 



To this cartilaginous cranium with its cartilage bones 

 certain membrane bones are added. Covering the roof 

 of the brain-case are two long narrow bones called the 

 fronto-parietals (FR. PA), because they are formed by the 



