FEOM THE KIMMERIDGE CLAY. 445 



by their ankylosis, the junction of the free ends of their lower trans- 

 verse processes, and the connexion of these with the ilinm) are four. 

 Although now disconnected (ankylosis not having yet occurred, owing 

 to the immaturity of the individual), the terminal surfaces of the 

 centra fit each other so truly that their natural sequence is not, I 

 think, open to doubt. 



With the anterior surface of the foremost sacral centrum arti- 

 culates another, in precisely the same manner as that in which the 

 true sacral centra are joined together. It is the last lumbar 

 vertebra, ISTo. 23. By the identity of its mode of union to the first 

 sacral centrum, and by the large support it affords to the first- sacral 

 lower transverse process, it so closely resembles the true sacral 

 vertebrae, and dynamically forms so obviously a part of the sacrum, 

 that Drs. Melville and Mantell were not very culpable in regarding 

 it as the first true sacral centrum. 



The last lumbar (PI. XX. figs. 1, 2, II) is bulkier than any of the 

 true sacral centra. Its form is depressed ; its anterior articular sur- 

 face is nearly plane, very slightly concave. It is smooth ; and it was 

 evidently capable of movement upon the next centrum in advance, to 

 which it was attached by an intervertebral disk of the ordinary 

 structure and form, the marks of which are still apparent. The ver- 

 tical diameter of this face is 52*5 millims., and the horizontal diameter 

 about 66 millims. Its posterior terminal surface is plane. It is 

 marked by radiating impressions suggestive of intimate union to the 

 next centrum by a thin film of ossifying cartilage. Its minimum 

 horizontal diameter in the plane of the neural canal is 40 millims., 

 and its maximum diameter, which is nearly on the level of its mid- 

 height, is 71 millims. It will be seen from a comparison of these 

 numbers how greatly the centrum expands behind. Here near 

 the posterior border the centrum attains the maximum horizontal 

 diameter of 91 millims., forming the anterior boundary of a deep 

 notch in the articulated sacrum between the last lumbar and first 

 sacral centrum, which aff'orded a very firm attachment to the first 

 lower sacral transverse process. The sides of the last lumbar 

 centrum, in the vertical direction, at first slope outwards from the 

 neural surface until the level of the lower limit of the notch just 

 described is reached. Prom here they bend rather abruptly in- 

 wards to an inferior median keel, at each side of which the surface 

 is transversely nearly plane. Longitudinally the sides of the 

 centrum are very concave ; the concavity is increased by the pro- 

 minence of the posterior margin. The neuro- central suture is 

 relatively shorter than in the other lumbar vertebrae ; the groove 

 of exit for the last lumbar nerve limits it behind. At a short 

 distance from its posterior limit the suture has a conspicuous in- 

 dentation, repeated in the last true sacral vertebra. 



The first true sacral centrum (jSTo. 24) is much smaller than the 

 last lumbar. Jxi front, at its junction with this, it is much ex- 

 panded, but towards its middle it rapidly contracts. The inferior 

 median keel, conspicuous in the last lumbar centrum, is here 

 scarcely noticeable. The form of the centrum is more cylindroid ; 



