528 Richard Owen, Esq., on the 



hardly be recognised as belonging to the genus Plesiosaurus : they are hol- 

 lowed out like the vertebrae of the Ichthyosauri, so as to join by double con- 

 cave surfaces, (as shown in the figure (PI. XLIV., fig. 6.) of the joints be- 

 tween the twenty-seventh, twenty-eighth, twenty-ninth, and thirtieth caudal 

 vertebrae of the skeleton of the Plesiosaurus Hawkinsii in the British 

 Museum). 



It is interesting to perceive that those vertebrae which are most remote from 

 the centre and source of vital energy, present throughout life a lower and 

 earlier type of structure ; such as, from the analogy of the Batrachia, we may 

 assume to have been the fcetal condition of all the other vertebrae. 



It is equally interesting to perceive the designed arrest of development at a 

 part of the spine where the elasticity and flexibility retained by that arrest are 

 required for its peculiar functions*. 



In conformity with the usual course of anatomical descriptions, we have 

 next to consider the bony hoops which encompass the visceral or thoracic- 

 abdominal cavity, and which consist of two parts, called the vertebral and 

 sternal ribs, although these are, in fact, respectively analogous to the costal 

 processes and haemapophyses in the caudal region of the spine, and ought 

 strictly to have been considered as parts of the dorsal vertebrae. The verte- 

 bral ribs are evidently but prolonged developments of the osseous appendages 

 of the transverse processes of the vertebree, which we have already seen to 

 exist in the cervical, sacral, and caudal regions of the spine. They begin 

 from the thirtieth cervical vertebra rapidly to increase in length and strength 

 to near the middle of the abdomen, and then more gradually diminish, and 



* The caudal haemapophyses have been hitherto described as chevron bones in the Plesiosaurus ; 

 but in all the specimens and casts which I have examined, they have presented the condition of 

 two appendages distinct from one another. In the PL Dolichodeirus at the British Museum, the 

 tail rests upon its inferior surface, and the broken spines and costal appendages are alone brought 

 into view. In the cast of the Duke of Buckingham's specimen, in the Museum of the Geological 

 Society, the only parts resembling chevron bones are the detached neurapophyses, or superior 

 arches. In Plate XXV. of PL Hawkinsii, in Hawkins's Memoir, the small chevron-shaped bone 

 below the fourteenth caudal vertebra, does not belong, as might at first sight be supposed, to the 

 under surface of that vertebra, but is the superior arch of the small, displaced vertebra behind it. 

 The true condition of the heemapophyses in the Plesiosaurus is beautifully shown at the sixteenth 

 caudal vertebra of the PL Hawkinsii in the British Museum. — (See Plate XXIV. of Hawkins's 

 Memoir.) The haemapophyses are longer and slenderer than the costal processes ; they slightly 

 taper to their distal extremities, which approach each other by a gentle curve, and were doubtless 

 connected together in the recent state by a cartilaginous or ligamentous substance. In the newly- 

 born whale, the hsemapophyses present the same condition, which, however, is a transitory one in 

 this warm-blooded marine animal, in which ossification proceeds to blend the haemapophyses into 

 a single chevron. 



