FOSSIL AND RECENT 93 



series would have first to take place, a view with little to recommend it. Alter- 

 natively, the evolution of the Pachythrissops-type from Allothrissops would imply 

 that the evolution of two sets of uroneurals occurred twice, independently and at the 

 same time. This latter view also seems untenable. Thus Pachythrissops, on caudal 

 fin anatomy alone, would appear distinct from the Allothrissops-Thrissops-ich.th.yo- 

 dectid lineage. 



Removal of Pachythrissops from association with Allothrissops and Thrissops 

 creates the vexatious problem of the true affinity of the genus. The resemblances 

 between P. propterus and Megalops mentioned by Nybelin (1964) may also be extended 

 to include the English species. Thus the premaxilla bears a band of fine teeth and 

 there are fulcra present on the upper caudal lobe. Other similarities may be found 

 in the lower jaw, where the oral border is concave and is produced to form an elongate 

 coronoid process. Certain resemblances may be seen in the hyopalatine series. The 

 hyomandibular and metapterygoid are similar in shape to those of the Megalopidae 

 and a prominent metapterygoid-endopterygoid ridge is present in the Recent mega- 

 lopids and Pachythrissops. The condyle of the quadrate is succeeded by a shallow 

 but well-marked cup which acts as a stop in the downward movement of the lower 

 jaw. 



The posterior face of the neurocranium, known only in P. vectensis, exhibits an 

 overall similarity with Tarpon. Thus the cranium is deep relative to its width, 

 the openings to the post-temporal fossae are large and the epiotic processes well 

 developed. In the Eocene and Recent megalopids the first vertebra is incorporated 

 with the neurocranium, the exoccipitals and basioccipital forming a tripartite 

 surface for it. In Pachythrissops the first vertebra is distinct and the condyle is 

 formed solely by the vertebra-like expansion of the basioccipital, as in Albula, 

 Pterothrissus and many other 'isospondyls'. In the skull roof certain features agree 

 with Tarpon ; the sphenotic spines are prominent and knob-like at their extremities, 

 the parietals are longer than broad, and the dilatator fossa appears virtually open 

 with only a narrow shelf of bone forming a roof. Conversely, there are several 

 differences in the proportions. The skull roof of Pachythrissops is narrow, the parie- 

 tals are ornamented posteriorly and several prominent ridges run longitudinally. 

 The parasphenoid shows the same form as in Tarpon, and, as far as can be seen, 

 is edentulous in contrast to the toothed vomer anteriorly. 



A single specimen of Pachythrissops laevis shows gill-rakers and scattered tooth 

 plates which have become detached from the branchial arch elements. In both 

 features a resemblance is noted with the megalopids. 



All these similarities are in primitive features and of little use as evidence of 

 natural relationship. An undisputed megalopid feature is the modification of the 

 intercalar. The intercalar is unmodified in Pachythrissops, yet this does not preclude 

 close phylogenetic relationship since it is postulated here that intercalar modification 

 was a relatively late (Eocene) development. As is so often the case one is forced to 

 rely on general appearance, and there is some agreement in superficial features. 

 Thus, Pachythrissops is tentatively placed in the Megalopidae but excluded from 

 discussion pending more complete knowledge, not only of the genus itself, but also 

 of contemporary teleosts. 



