66 



ELOPIFORM FISHES 



soc 



exo 



ot. s c 



par 



Fig. 31. Megalops cyprinoides (Broussonet). Neurocranium in posterior view. 

 Based on same specimen as Text-fig. 30. 



The neurocranium of Megalops is relatively deeper and the maximum depth occurs 

 at the sphenotic level, whereas in Tarpon the maximum depth is at the occiput. The 

 depth of the Megalops neurocranium is due to the depth of the prootic bone and 

 parasphenoid ascending wings which in turn may be related to the invasion of the 

 skull by the swimbladder in the following manner. Megalops has a large eye, 

 relatively larger than that in Tarpon, and thus might be expected to have a large 

 posterior myodome. The extreme anterior end of the swimbladder diverticulum is 

 received in a deep pit within the prootic bone immediately above the prootic bridge, 

 which is, in consequence, displaced ventrally. This lowering of the myodome roof 

 would restrict its volume. Hence it is suggested that the myodome volume is 

 maintained by effectively lowering its floor, i.e. by deepening the prootic and para- 

 sphenoid walls. Correlated with this deepening is the angled parasphenoid which 

 is in contrast to the relatively straight bone seen in Tarpon. 



The intercalar bone of Megalops has been well described by Greenwood (1970a), 

 who recognized three interconnected portions ; a basal portion, a lateral shield-like 

 part and a saddle-like wing. The lateral shield-like portion forms a complete bony 

 outer wall to the periotic bulla and is joined to the prootic, basioccipital and exoc- 

 cipital by a syndesmotic union. The anterior swimbladder diverticulum occupies 



