FOSSIL AND RECENT 109 



Very few specimens of 0. lewesiensis show the dermal rays of the caudal fin ; 

 none shows a complete series and thus the number, nature, length and distribution 

 are unknown. The following description of the caudal skeleton is based on B.M.N.H. 

 49894. 



The fin-rays are supported by structures associated with four preural and two 

 ural centra. The latter are distinguished from those anteriorly by being unorna- 

 mented, upturned and longer than they are deep. 



The haemal arches borne by the first four preural centra are firmly united, although 

 not fused, with the centra. The haemal spines remain broad throughout their length 

 and are thickened along their posterior margins. The anterior margin is produced 

 as a thin wing which lies against the preceding spine. Near to the base of the spine 

 the anterior margin is notched for the passage of the segmental artery. 



The third and fourth preural centra bear full length neural spines which converge 

 distally. The second preural centrum bears a neural arch and a half length spine, 

 while the first preural bears only a neural arch, the outline of which is difficult to see. 

 All neural ar.m elements are autogenous. 



The hypural series forms a complete fan. Two lower hypurals are borne by the 

 first ural centrum. The lowermost or first hypural is by far the larger of the two 

 and is fan-shaped, in contrast to the parallel-sided second element. The bases of 

 these hypurals are slightly constricted and produced into rounded heads which fit 

 in weakly defined cup-shaped depressions on the supporting centrum. 



Three upper hypurals may be seen (though it is very likely that there was a fourth 

 in the series representing the ' hypurale minimum ' of Monod 1967). The third 

 hypural, the lowermost member of the upper series, is large and fan-shaped. It is 

 divisible into a thickened upper and a thin lower portion. The fourth hypural is 

 considerably narrower, but similar to its ventral neighbour in articulating with the 

 second ural centrum. 



There are four uroneural elements. The first is expanded proximally and covers 

 the dorso-lateral surface of the first and second preural centra. The second uroneural, 

 which in the specimen figured (Text-fig. 53) is displaced posteriorly, is slightly 

 smaller and lies in a groove on the posterior face of the first uroneural. The third 

 lies in conformity with the second but the fourth appears to have been displaced 

 posteriorly. The nature of this fourth element is in question. Its size and shape 

 would suggest that it represents a urodermal, but its position within the fin-rays, 

 not superficial to them, and the nature of the bone surface are features of a uroneural. 

 This structure is interpreted as being a fourth uroneural. 



Between the first uroneural and the second preural neural spine there are three 

 epurals. The posterior epural is the largest. The anterior epural is markedly 

 curved but it is not clear how much of this curvature is due to the distortion which 

 has obviously taken place. 



The base of the inner ray of the upper lobe shows a marked basal expansion as in 

 many lower teleosts. A caudal scute was present both above and below the caudal 

 peduncle. 



Squamation. A lateral line scale count cannot be made. The type specimen 

 exhibits 38 scales to the level of the pelvic fin insertion. If the pelvic fin origin be 



