i 4 6 ELOPIFORM FISHES 



Cephalic sensory canal system. The sensory canals of the head were mainly con- 

 tained within open troughs. A comparison of Istieus with Pterothrissus shows a 

 similarity in basic design. On the skull roof the supraorbital canal of Istieus runs 

 for a greater distance within the frontal, opening to the surface of the bone above 

 the anterior half of the orbit. The otic sensory canal of Istieus is contained within 

 a groove, the walls of which are more complete than in Pterothrissus. 



Hyoid arch and branchiostegal rays. The anterior ceratohyal of Istieus is relatively 

 longer than in Pterothrissus and this is reflected in a greater number of branchiostegal 

 rays. 8-10 branchiostegals are present in Istieus, the upper (posterior) two being 

 borne by the posterior ceratohyal. The variation in the number of branchiostegals 

 affects those placed anteriorly. 



Each branchiostegal is broad, curved and marked by a prominent ridge which 

 expands proximally to form an articular head. 



Poster anial skeleton. The impressions left by the pectoral girdle and fin are 

 imperfect but the observable details are similar to comparable structures in Ptero- 

 thrissus. The supratemporal is small and placed laterally, the cleithrum shows a 

 posterior expansion above the area of fin insertion and the anterior portion of the 

 cleithrum is narrow. The fin is composed of not less than 12 rays. In outline the 

 fin is rounded distally. A pectoral splint is associated with the base of the outermost 

 ray. 



The pelvic girdle and fin are closely similar to those of Pterothrissus. The pelvic 

 fin originates beneath the thirty-eighth to forty-second vertebra. 



The vertebral count varies from 87 to 92 in the few specimens from which reasonably 

 accurate counts could be made. The posterior 38-40 vertebrae are caudal, giving a 

 caudal/total ratio of 48 per cent, a figure considerably higher than in Pterothrissus 

 gissu in which this ratio is 32 per cent (Istieus macrocephalus has a caudal/total ratio 

 of 40 per cent). Even though there are relatively more caudal vertebrae in /. 

 grandis, the relative length of the caudal region is the same as in Pterothrissus, 

 a discrepancy due to the majority of the caudal centra in /. grandis being 

 shorter than the abdominal centra. /. macrocephalus, although having relatively 

 fewer caudal vertebrae than I. grandis, shows a relatively longer caudal region than 

 either I. grandis or Pterothrissus. The abdominal centra of /. macrocephalus are 

 considerably shorter than the caudal centra, the reverse of the situation in /. grandis. 



The morphology of the neural arches and spines, the haemal arches and spines, 

 the parapophyses, pleural ribs, intermusculars and supraneurals is similar to that 

 described for Pterothrissus. In some specimens of Istieus irregularly shaped ridges 

 are seen passing upwards and backwards from the bases of the neural arches. These 

 ridges are interpreted as the impressions left by tough myocommata, a feature noted 

 in Pterothrissus. 



Both the dorsal and anal fins are similar in shape, relative size and anatomy to 

 those of Pterothrissus. The differences are merely those of meristic counts. 



The caudal skeleton of I. grandis is similar to Pterothrissus as may be seen by 

 comparing Text-figs. 72 and 68. The important points of similarity are : the orna- 

 mentation of the centra, the narrow neural and haemal spines, the two narrow uro- 

 neurals, the narrow hypurals with a large gap between the second and third, the 



