88 ELOPIFORM FISHES 



The scales are relatively small, unlike those of megalopids, and are thin and 

 cycloid. The presence of bone cells has not yet been demonstrated. The anterior 

 field is marked by 4-5 radii resulting in 3-4 scallops along the anterior margin. 

 Tarpon usually exhibits 4 radii, while Megalops shows about 6. Elops and Davich- 

 thys scales show at least 11 radii. The ornamentation of the exposed field is like the 

 Eocene megalopids in showing a fine granulation composed of minute tubercles. 

 A point of distinction is that the granulations in Sedenhorstia appear to be arranged 

 in radiating lines. The exposed surface of elopid scales is unornamented except for 

 the area immediately adjacent to the nucleus. 



It is regrettable that the ethmoid and intercalar of Sedenhorstia are not well known, 

 for it is in these areas that the megalopids are most clearly characterized. Never- 

 theless, the features referred to above serve to establish that Sedenhorstia exhibited 

 a morphology comparable with that of the Megalopidae. Features of the dentition 

 and perhaps the peculiar ossifications in the dorsal ligament divorce Sedenhorstia 

 from the main megalopid line, but its close alliance with this lineage seems certain. 

 The phylogenetic position of Sedenhorstia is further analysed in the general dis- 

 cussion of the Megalopidae (p. 201). 



Genus PACHYTHRISSOPS Woodward, 1919 



? 1914 Parathrissops Eastman : 423 (name preoccupied : Parathrissops Sauvage, 1891 : 37). 

 1 919 Pachythrissops Woodward : 128. 



Diagnosis. See Woodward (1919). 



Remarks. The genus is briefly discussed following a proposal by Nybelin (1964) 

 that at least one of the contained species shows a resemblance to Megalops. 



The genus Pachythrissops is known from three species. The type-species, P. 

 laevis Woodward, is from the Upper Portlandian of Dorset, England ; P. vectensis 

 Woodward is from the Weald Clay (?L. Aptian) of the Isle of Wight, England, and 

 P. propterus (Wagner) is from the Kimmeridgian of Solenhofen, Germany. Question- 

 ably, the so-called Parathrissops furcatus Eastman is also placed in this genus. 



Several complete specimens of the type-species were examined together with two 

 specimens of P. vectensis and one specimen (B.M. N.H. 37056) which is probably 

 a young individual of P. propterus. 



Discussion. During a revision of the genus Thrissops Agassiz, Nybelin (1964) 

 briefly described P. propterus and suggested that it was only remotely related to 

 Thrissops and Allothrissops. The latter genera are considered to be related to the 

 Ichthyodectidae. Furthermore, Nybelin tentatively proposed that P. propterus 

 may be an ancestor of Megalops, quoting similarities in premaxillary dentition, 

 caudal fin fulcra and other unspecified characters. 



Bardack (1965) included Pachythrissops (recognizing the type-species as P. 

 propterus) in the family Chirocentridae (sensu Saint-Seine 1949) and endorsed 

 Nybelin's views, quoting the presence of a gular plate in P. propterus as further 

 evidence for separating this species from Allothrissops and Thrissops. Bardack 



