xm PHYLUM CHORDATA 307 



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 conveni- 

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

 having ribs connected with the sternum. 



The sternum (Fig. 974, 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 skull (Fig. 973) the chondrocraniurn, though persistent, 

 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 (has. oc.) 

 below, exoccipitals (ex. oc.) at the sides, and a supraoccipital (supr. oc.) 

 above. The basioccipital forms the floor of the most posterior 

 portion of the cranial cavity ; posteriorly it bears a rounded promi- 

 nence, the occipital condyle (oc. cond). In front of it, forming the 

 middle portion of the floor of the cranial cavity, is the basisphenoid 

 (has. sph), not represented in the Frog, in front of which again is an 

 investing bone, the parasphenoid (para), corresponding to the bone 

 of the same name in the Frog and Trout, but here much reduced in 

 size and importance and ankylosed with the basisphenoid. 



In the wall of the auditory capsule are three ossifications 

 pro-otic, epiotic, and opisthotic (op. ot). 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 interorbital 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 foramen (par. f). The frontals remain separated from 

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

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

 roofed over by a pair of nasals (nas). A small pre-frontal (pr. fr.) 

 lies in front of the frontal, and helps to bound the orbit anteriorly, 

 and another small bone the lacrymal (kr) perforated by an aper- 

 ture for the lacrymal duct, lies at the anterior extremity of the orbit, 



