ON BRITISH FOSSIL REPTILES. 115 



length, and descend from the neurapophysis to the centrum as the vertebrs 

 approach the end of the tail. 



The chevron bones also decrease in length, but they expand in the antero- 

 posterior direction at their unattached and dependent extremity, M'hich is 

 defined by a slight convex outline. The following admeasurements give the 

 rate of decrease in length in the caudal vertebras, taken at intervals of six 

 joints : — In. Lines. 



Length of body of presumed 8th caudal 2 6 



Length of body of presumed 14th caudal 2 ^ 



Length of body of pi-esumed 20th caudal 2 2 



The sides of the slender posterior vertebrae are distinguished by a slight 

 median expansion below the base of the rudimental transverse jDrocess, so that 

 the surface, instead of being gently concave lengthwise, undulates by virtue 

 of the middle elevation. I have not met with this character in the corre- 

 sponding vertebrae of other Saurians. In the vertical direction the sides of 

 the centrum in the posterior caudals converge at almost a right angle to the 

 inferior groove. The greater breadth of the centrum, in proportion to its 

 height, may still be discerned in the terminal caudal vertebrae : thus in the 

 centrum 2 inches 2 lines long, the breadth was 1 inch 10 lines, and the 

 height only 1 inch 3 lines. 



Dermal scutes. — Unequivocal evidence that a dermal skeleton, analogous 

 to that in the recent Crocodiles, was developed in the Hylceosaurus, was af- 

 foi'ded by the discovery of bony scutes in the mass of vegetable matter re- 

 moved in clearing the portion of the skeleton first described. Some of these 

 detached bony plates still adhere to the caudal vertebrae, and may be observed 

 to decrease in size as they approach the end of the tail. From their form, 

 which is elliptical or circular, and from the absence of any surface indicating 

 the overlapping of an adjoining scute, it may be inferred, that the bony 

 plates in question studded in an unconnected order the skin of the Hylaeo- 

 saur. The diameter of the largest of these scutes does not exceed three 

 inches ; the smallest present a diameter of one inch. They are flat on the 

 under surface, convex with the summit developed into a tubercle in the smaller 

 specimens, but which is less prominent in the larger ones : the outer surface is 

 studded all over by very small tubercles : the inner surface presents the fine 

 decussating straight lines, already noticed in the scutes of the Goniopholis. 



By the kindness of Dr. Mantell, I have been favoured with the means of 

 submitting the structure of a dermal scute to microscopical examination. 



The medullary canals, which are stained brown, as if with the hematosine 

 of the old reptile, differ from those of ordinary bone in the paucity or absence 

 of concentric layers. They are situated in the interspaces of straight opake 

 decussated filaments, which frequently seem to be cut short off close to the 

 medullary canals. Very fine lines may be observed to radiate from some of 

 the medullary canals : irregularly shaped, oblong and angular radiated cells 

 are scattered through most parts of the osseous tissue, but they present less 

 uniformity of size than do the Purkinjian cells in ordinary bone. The most 

 striking characteristics of the dermal bone are the long straight spicular fibres 

 which traverse it, and decussate each other in all directions, representing, as 

 it seems, the ossified ligamentous fibies of the original corium. 



Dermal spines? — On the left side of the thorax, partly overlying the left 

 scapula and vertebral ribs, in the large slab of stone containing the anterior 

 part of the skeleton, there are some large elongated, flattened pointed plates 

 of bone, three of which seem to follow each other in natural succession. The 

 length of the first of these plates is seventeen inches, the breadth of the base 

 five inches, equal to the antero-posterior diameter of two vertebrae : they de- 



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