78 ELOPIFORM FISHES 



Protarpon oblongus (Woodward) 



1844 Hypsodon oblongus Agassiz : 5, pt. 1 : 101 {nom. nud.). 



1901 Megalops oblongus Woodward : 26. 



1966 Megalops oblongus Woodward ; Casier : 119, pi. 13, fig. 2. 



Diagnosis. See Woodward (1901 : 26). 



Holotype. B.M.N.H. P. 634, an otic region of a neurocranium, rather poorly 

 preserved. 



Material. Only the holotype is known. 



Formation and locality. London Clay (Ypresian) of Sheppey, Kent, England. 



Remarks. This species is retained with reservation. Woodward (1901) dis- 

 tinguished it on the basis that the distance from the occiput to the level of the 

 sphenotic spines is equal to the width at the sphenotics. Although the precise 

 distance between the sphenotic spines cannot be measured in B.M.N.H. P. 634 

 (the lateral roof margins are broken in the specimen) Woodward's distinguishing 

 character remains essentially true. 



In the Recent Tarpon the proportions of the posterior part of the skull roof change 

 slightly throughout ontogeny, there being a slight increase in length relative to width. 

 However, in P. prisons the occiput-sphenotic distance never exceeds 75 per cent of 

 the intersphenotic distance, and is thus very different from that seen in P. oblongus. 



Casier (1966 : 119) cites the more weakly developed frontal depression and the 

 convergence of the ridges on the skull roof as further differences from P. prisons, but 

 in view of the great variability of these features in the genus Protarpon it seems best 

 to ignore them. 



The parietal bones of P. oblongus are only partially preserved, yet enough remains 

 to indicate that their length exceeded twice their width, as in Tarpon, whereas the 

 parietals of P. prisons are only slightly longer than wide. However during the 

 growth of Tarpon the shape of the parietals changes from square to rectangular. 

 The lateral otic wall of the neurocranium is very similar to that of P. prisons. 



In view of the ontogenetic changes seen in Tarpon it could be argued that P. 

 oblongus is merely an older individual of P. prisons, but the small difference in size 

 between P. prisons and P. oblongus could not account for the considerable differences 

 in proportions mentioned above. Irrespective of the true specific identity, the 

 important fact remains that the otic region is very similar to P. prisons in the roofed 

 dilatator fossa, hyomandibular facet, large subtemporal fossa and the position of the 

 vagus and jugular foramina. 



Discussion of the genus Protarpon 



Protarpon is clearly a member of the Megalopidae. With these fishes it shares 

 such characters as a deep neurocranium with an otic depression comparable to that 

 receiving the swimbladder in Recent forms ; very large post-temporal fossae ; 

 large, prominent epiotic bones ; narrow, virtually edentulous parasphenoid ; and 

 general characters of the ethmoid region (preservation of this region precludes 



