xni PHYLUM CHORDATA 295 



by a ligamentous band from the upper portion, which corre- 

 sponds to the neural arch, and lodges the anterior end of the 

 spinal cord. On the ventral surface of the axis, and of each of 

 the following five or six vertebrae, is a distinct bony nodule, 

 sometimes termed the hypapoplvysis (hyp). The sacral vertebrae 

 have short centra and strong expanded processes the transverse 

 processes which abut against the ilia ; these are separately 

 ossified, and are to be looked upon as sacral ribs. The anterior 

 caudal vertebra are like the sacral, but have the centra longer, 

 the transverse processes more slender, and the neural spines 

 longer. The posterior caudal vertebrae become gradually smaller 

 as we pass backwards, and the various processes reduced in pro- 

 minence, until, when we get to the end of the tail, the whole 

 vertebra is represented merely by a rod-like centrum. Attached 

 to the ventral faces of the centra of a number of the anterior caudal 

 vertebrae are Y-shaped bones the chevron bones the upper limb 

 of the Y articulating with the vertebra, while the lower limb 

 extends downwards and backwards. In nearly all the caudal 

 vertebrae the centrum is crossed by a narrow transverse unossified 

 zone through which the vertebra readily breaks. The ribs are 

 slender curved rods, the vertebral end of each of which articu- 

 lates with one of the capitular facets of the corresponding vertebra. 

 The ribs of the five anterior thoracic vertebrae are connected by 

 means of cartilaginous sternal ribs with the 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 dis- 

 tinguish 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. 908, sf) 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. 907) the chondrocranium, though persistent, 

 is replaced by cartilage bones to a much greater extent than in 

 the Frog, and the number of membrane 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 

 basi-occipital (has. oc) below, ex-occipitals (ex. oc) at the sides 

 and supra-occipital (supr. oc) above. The basi-occipital forms the 



