INTRODUCTION. xiii 



thickened at their upper end. The twenty-eighth vertebra (at the bend) has a high 

 neural spine thickened at its upper end and upright or a little inclined forwards. 

 The spines of the succeeding five vertebrae are inclined forwards and gradually become 

 shorter. On the next the short spine slopes backwards, and the same is the case with 

 the next two. Behind this to the end of the tail — that is, in about the last fifteen 

 vertebrae — no spines can be seen, and probably in some of the terminal vertebra? 

 no neural arch was present. 



The chevrons begin on the second or third caudal vertebra ; at first they are long 

 Y-shaped bones, but from the fourteenth to the twenty-fifth vertebne they are not 

 clearly seen. Behind this they have very short pedicles and a much expanded median 

 ventral portion, consisting of a smaller anterior and a longer posterior lobe. The ends 

 of these lobes in the successive chevrons are in contact, forming a continuous chain 



Text-fig. B. 



mc.+. 



o a 



Fore paddle of Geosawus gracilis. (R. 3948, nat. size.) 

 a., radius; 6., ulna; c, radiale ; d., ulnare; mc.l, first metacarpal; mcA, fourth metacarpal. 



as far back as the thirty-seventh vertebra : this arrangement probably greatly 

 strengthened the basal portion of the tail-fin. 



The structure of the shoulder-girdle and fore limb can, to a great extent, be made 

 out from their impressions on the matrix. The fan-shaped upper end of a coracoid 

 with a coracoid foramen is well shown, and there is a mass of bone crushed beneath 

 the vertebral column, which may be the lower part of a scapula, but the state of 

 preservation is such that this is uncertain. The humerus is wanting, but the form of the 

 paddle-like distal part of the limb is well shown ; this is figured above (text-fig. B). 

 It will be seen that it consists proximally of two pairs of subequal disc-like bones, 

 the nature of which is not quite clear. Ammon, in his description, regards the 

 proximal pair (a., b.) as the radius and ulna, the distal (c, d.) as the radiale and ulnare. 



