FOSSIL AND RECENT 45 



There is a great deal of literature on both locality data and growth stages of T. 

 atlanticus, a useful synopsis of which is found in Hildebrand (1963). 



Remarks. The skeleton of Tarpon atlanticus has never been satisfactorily de- 

 scribed. This, and the fact that in several respects it is more primitive than the 

 frequently cited Megalops cyprinoides has led to the inclusion here of a more complete 

 account. Omitted details are those of the caudal fin anatomy, which have been dealt 

 with by Hollister (1936 : 263-268), and the otic region of the skull (in so far as this 

 has been affected by the anterior swimbladder diverticula) which has been described 

 by Greenwood (1970a). 



Description. In a young individual (c. 87 mm S.L.) the cranium is one and a 

 half times as long as deep. Throughout life the depth increases with respect to 

 length. Other morphometric changes that occur during growth are dealt with in 

 the description. Unless otherwise stated, remarks apply to adult individuals. 



Neurocranium. The skull roof is flat anteriorly but becomes strongly convex 

 posteriorly. In lateral view the dorsal margin of the skull roof is perfectly straight, 

 or in some large specimens may even exhibit a slight concavity (cf. Gregory 1933 : 

 fig. 31). The greatest width of the neurocranium is equal to 60 per cent of its 

 length and occurs at the sphenotic level. During growth the skull roof becomes 

 relatively wider, due primarily to the development of strong sphenotic spines, and the 

 distance from the occiput to the level of the sphenotic spines increases relative to 

 the total neurocranial length. This differential growth is quite considerable (in a 

 fish of c. 87 mm S.L. the distance from the occiput to the level of the sphenotic 

 spines is 26-5 per cent of the total neurocranial length, while in a fish of 650 mm this 

 distance is 37 per cent) and may reflect the development of the enormous post- 

 temporal fossae. 



The anterior portion of the skull roof is formed, as in all megalopids, by a complex 

 median dermethmoid. The horizontal part of this bone is constricted one-third of 

 the way back. To the anterior margin of the bone is attached a strong ligament 

 from the head of the maxilla. The ethmoid commissure runs transversely through 

 the tubular rostral, near the anterior border. Laterally, a large pore receives the 

 canal from the lateral rostral. Immediately medial to this, another large pore 

 opens posteriorly and near the mid-line a much smaller pore opens anteriorly. 

 Posteriorly, the dermethmoid overlaps the anterior end of the frontals. A postero- 

 ventrally directed wing arises from beneath the lateral border of the dermethmoid 

 and passes down over the cartilage surrounding the olfactory organ. Posteriorly 

 this wing, in older specimens, is in sutural contact with the lateral ethmoid. 



It appears that the lateral processes are ossifications embryonically distinct from 

 the main body of the dermethmoid. Patterson (1970b) states that similar ossifica- 

 tions are present in Leptolepis coryphaenoides and Megalops and that they are similar 

 to the ' proethmoids ' of the stomiatoid Polymetme. For a fuller account of the 

 homology of these lateral processes see Patterson (1970b : 261-262). 



The frontal is the largest single component of the skull roof. Anteriorly it passes 

 beneath the dermethmoid almost to the anterior end of the latter (Nybelin 1967a : 

 fig. 2C figures the anterior termination of the frontal as lying completely behind the 

 dermethmoid), while posteriorly the frontal overlies part of both the pt erotic and 



