BRITISH FOSSIL REPTILES. G5 



above and the centrum below. But in other cervical vertebrae 

 of a still larger Plesiosaur in the collection of Lord Cole, not 

 only is the spine anchylosed with the neurapophyses, but these 

 are also confluent with the centrum. 



In the dorsal region in the PL inacrocephaius, as in the PL 

 Hawkinsii and PL dolichodeirus, the neur apophyses and spines 

 become anchylosed ; but the former elements continue separate 

 from the body of the vertebra throughout the vertebral column 

 in the PL macrocephalus. 



The cervical spines in the PL macrocephalus differ in form 

 from those of PL Hawkinsii in retaining their breadth or an- 

 tero- posterior extent throughout the neck ; their extremities 

 being, as it were, truncate, with the angles slightly rounded off. 

 The powerful ridge of bone which they thus collectively form 

 is highly characteristic of this species. The consequence of 

 this structure is a diminution of the spinal interspaces necessary 

 for the vertical inflections of the neck ; which interspaces are 

 conspicuously present in the PL Hawkinsii, where the end of 

 each cervical spine is as it were obliquely cut off at the ante- 

 rior part, so as to allow the neck to be bent upwards much 

 more extensively than could have been possible in the PL ma- 

 crocephalus. What, however, the latter species thus lost in 

 mobility it gained in strength, the quality mainly required in 

 relation to the movements of its more bulky and ponderous head 

 and jaws. 



As the powerful neck of the PL macrocephalus, however, 

 possessed extensive mobility in the lateral direction, as is indi- 

 cated by its position in Lord Cole's fossil, the muscles destined 

 for these movements must necessarily have been developed in a 

 corresponding degree ; and we find that adequate provision was 

 made for their fixed points of action, in the superior develop- 

 ment of the costal processes, as compared with those of PL 

 Hawkinsii : these processes present, indeed, throughout a 

 greater part of the neck the clfaracteristic expansion of their 

 distal extremities, which led to their being called hatchet- 

 shaped bones by Mr. Conybeare : but the stem which supports 

 the dilated extremity is proportionally longer in the PL macro- 

 cephalus-, and it is only towards the baie of the neck that 

 the extremities overlap each other, as in the Crocodile. Dr. 

 Buckland has illustrated this peculiarity by placing side by side 

 the figures of the hatchet bones in the PL Hawkinsii and PL 

 macrocephalus in his Bridgew^ater Treatise. These cervical 

 ribs assume the true costal form, as before stated, at the twenty- 

 seventh vertebra, where they are short and straight ; behind tins 

 VOL. VIII. 1839. F 



