FOSSIL AND RECENT 181 



notch in the posterior margin. This notch lies in juxtaposition to a notch in the 

 anterior margin of the first hypural and marks the divergence of two lateral caudal 

 arteries from the median dorsal aorta. 



Dorsally, the neural spines associated with the second to fifth preural centra are 

 slender, each with a stout posterior edge preceded by a thin median wing. The tips 

 of the last three neural spines converge to support the anterior dorsal accessory rays. 

 The neural arch complex associated with the first preural centrum appears to be a 

 double structure, i.e. two neural arches set one behind the other. This arrangement 

 is open to three interpretations. It may represent the two lateral halves of a single 

 first preural neural arch, these having become displaced relative to one another 

 during preservation (in this connection it is interesting to note that in one specimen 

 of Pterothrissus examined the two halves of this arch remain distinct from one 

 another except at their extreme tips). Secondly, it may represent a double neural 

 arch upon this centrum ; such a condition is quite common in lower teleosts and has 

 been recorded for Tarpon (Hollister 1936 : fig. 16). The third interpretation is that 

 the posterior structure is the neural arch of the first ural centrum which has become 

 displaced anteriorly (cf. Nybelin 1971). Examination of other specimens failed to 

 substantiate any of these interpretations. 



There are at least two uroneurals. The first is large and shows a forked base 

 overlapping the first and part of the second preural centrum. The second uroneural 

 is long, extending from a lateral overlap of the first ural centrum to well beyond the 

 distal extremity of the first uroneural (a typical albuloid disposition). Specimen 

 B.M.N.H. P. 4856 shows a small element which overlaps the dorsal extremity of the 

 second uroneural. Its appearance suggests a uroneural, yet the bone, which shows 

 no sign of having been broken, is truncated, unlike a uroneural. Distally this ele- 

 ment reaches between the bases of the posterior dorsal basal fulcra. 



The first and second hypurals are supported by the first ural centrum. Distally 

 the hypurals support the principal fin-rays of the lower caudal lobe. As is usual 

 among lower ' isospondyls ' the first hypural is the larger and has a characteristically 

 narrow articular head. In the specimen figured the lower hypurals have been slightly 

 displaced postero-ventrally from their true position. 



Four upper hypurals support the principal rays of the upper lobe. The third 

 hypural is large and is the only member of the upper series showing a distinct ar- 

 ticulatory head. The uppermost hypural is very small and may represent a fusion 

 of two elements since there is a faint line of division running down the length of 

 this hypural. In the specimen figured the upper hypurals have been displaced 

 antero-ventrally. 



Between the first uroneural and the last neural spine there are three epurals, the 

 distal extremities of which are obscured by the bases of the basal fulcra which they 

 support. 



There are 19 principal rays, 10 in the upper lobe and 9 in the lower. The upper- 

 most principal ray is large, unbranched and does not extend over any endochondral 

 element (cf. Patterson 1968a). The innermost principal rays do not show expanded 

 bases and are thus similar to those of Recent albuloids. The lowermost principal 

 ray is supported by the parhypural and the second preural haemal spine. 



