XIII PHYLUM CHORD ATA .'^ir 



the five anterior thoracic vertebra? are connected by means of 

 cartilaginous sternal ribs with tlie sternum. The posterior thoracic 

 ribs do not reach the sternum, the sternal ribs being very short, 

 and free at their ventral ends. The cervical ribs, which are 

 present on all the cervical vertebrae with the exception of the first 

 three, are all shorter than the thoracic ribs, and none of them are 

 connected with the sternum. Thus, as regards the structure of 

 the vertebrae themselves, there is nothing to distinguish the 

 posterior cervical from the anterior thoracic ; but, for convenience 

 of description, the first thoracic is defined as the first vertebra 

 having ribs connected with the sternum. * 



The sternum (Fig. 963, st) is a rhomboidal plate of cartilage with 

 a small central space, or fontanelle, completed by membrane. 

 Posteriorly it is produced into two slender flattened processes. 

 On its antero- lateral borders are articular surfaces for the bones 

 of the pectoral arch, and on its postero-lateral borders and the 

 processes are small facets for the sternal ribs. 



In the skidl (Fig. 962) the chondrocranium, though per- 

 sistent, is replaced by bones to a much greater extent than in 

 the Frog, and the number of investing bones is much greater. 

 On the dorsal and lateral surface are a large number of dermal 

 roofing bones. At the posterior end the rounded aperture of 

 the foramen magnum {for. mag.) is surrounded by four bones — 

 a basioccipital (bas. oc.) below, exoccipitals {ex. oc.) at the sides 

 and a swpraoccipital {supr. cc.) above. The basioccipital forms the 

 floor of the most posterior portion of the cranial cavity ; posteriorly 

 it bears a rounded prominence, the occipital condyle {oc. cond). 

 In front of it, forming the middle portion of the floor of the cranial 

 cavity, is the basisphenoid (bas. sph), not represented in the Frog, 

 in front of which again is an investing bone, the parasplicnoid 

 {para), corresponding to the bone of the same name in the Frog, 

 and Trout, but here much reduced in size and imporfance and 

 ankylosed with the basisphenoid. 



In the wall of the auditory capsule are three ossifications — 

 pro-otic, epiotic and opisthotic {op. of). The first remains distinct, 

 the second becomes merged in the supraoccipital, and the third 

 in the exoccipital. The exoccipital and opisthotic are produced 

 outwards as a pair of prominent horizontal processes, the parotic 

 processes. 



The large orbits are closely approximated, being separated 

 only by a thin vertical interorbitcd septum. The cranial cavity 

 is roofed over by the parietals {par) and frontals {fr). The former 

 are united together ; in the middle is a small rounded aperture — 

 'the parietal forctmen {peer. /). The frontals remain separated from 

 one another by a median frontal suture : between them and the 

 united parietals is a transverse coronal sutxire. The nasal cavities are 

 roofed over by a pair of nascds {necs). A smuh pre-frontcd {pr.fr.) 



