8 FOSSIL REPTILIA OF THE LONDON CLAY. 



then, which we have to notice, is marked in figs. 4, 5, and 6, pi, signifying ' pleura- 

 pophysis,' the name of these elements. In the segments figured they retain their 

 primitive distinctness, and acquire unusual length, in order to aid in encompassing the 

 dilated canal or eavity for the heart and lungs : so modified, these elements are 

 commonly called ' rihs,' or ' vertebral ribs.' 



The elements more constantly employed to protect the vascular or ' haemal' axis, in 

 other words, to form the inferior or haemal canal, are those marked h in figs. 4 and 6 : 

 they are the ' hsemapophyses,' which are usually articulated, like the neurapophyses, 

 with the centrum, but are displaced by the great centres of the vascular system in the 

 thorax, where they have got the special name of ' sternal ribs,' and also that of ' costal 

 cartilages,' or ' cartilages of the ribs,' when they do not become ossified. The haemal 

 arch in the thorax is usually completed by a median element (/is), called a ' haemal 

 spine,' but which itself becomes vastly expanded in the bird (fig. 4) ; it is, nevertheless, 

 the part in the haemal arch which repeats below, or answers to the part (ns) in the 

 upper arch. In the segments of the trunk and tail, the element (ns) retains its normal 

 size and form as a ' neural spine ;' but where the central axis of the nervous system 

 becomes unusually developed, as in the head, e. g., analogously to the development of 

 the vascular centres in the chest, the neural canal is correspondingly expanded, and the 

 cavity acquires a special name, and is called ' cranium,' just as the analogously expanded 

 hseinal canal is called ' thorax.' Into the formation of the wall of the cranium other 

 vertebral elements enter besides the neurapophyses, those e. g. which are numbered 8 

 and 12 in fig. 9, p. 17, of the present Part ; the neural spine (7 and 11 in the same figure) 

 retains its primitive distinctness, is expanded horizontally, and, like the ' sternum' in the 

 thorax of the bird (/is, fig. 4, p. 5, Part I), it receives a special name (7), e. g. of ' parietal', 

 and (9) of ' frontal' in fig. 9. The elements a a (figs. 4 and 6, Part I) form a symmetrical 

 pair of bones or cartilages, attached at one end to the haemal arch, and projecting out- 

 wards and backwards. These are the ' prosartemata,' or appendages ; they are, of all the 

 elements of the vertebral segment, those that are least constant in regard to their 

 presence, and, when present, are subject to the greatest amount of development and 

 metamorphosis : they become, e. g., the pterygoid appendages in the nasal segment of the 

 fish's skull, the opercular bones in the frontal segment of the fish, the branchiostegal 

 rays in the parietal segment, the pectoral fins in the occipital segment, and they are deve- 

 loped into the fore limbs and hind limbs, the arms, wings, and legs of other Vertebrata.* 



As the nervous and vascular centres become reduced in size, the bony canals or 

 arches protecting them are simplified and contracted, and the vertebra assumes a 

 symmetrical character. In the Crocodile, the haemal arch, in the tail, e. g., is formed 

 by the hsemapophyses, which ascend and articulate directly with the centrum ; 

 the pleurapophyses are shortened, directed outwards, and become anchylosed to 



* The facts and arguments in support of this conclusion, are detailed in my works ' On the Nature of 

 Limbs,' and ' On the Archetype of the Vertebrate Skeleton,' 8vo (Van Voorst). 



