i 4 4 ELOPIFORM FISHES 



relationships of the metapterygoid to the endopterygoid and ectopterygoid are 

 similar to those in Pterothrissus. The ectopterygoid bears few teeth and these are 

 considerably smaller than those on the endopterygoid or parasphenoid. The quad- 

 rate differs from that of Pterothrissus in being without a posteriorly directed spine. 

 The palatine was cartilaginous and covered anteriorly by a small dermopalatine. 



Dermal upper jaw. The upper jaw extends back to behind the level of the lateral 

 ethmoid, in contrast to that of Pterothrissus. In shape and mode of articulation 

 both the premaxilla and maxilla are similar to those in the Recent genus. However, 

 the maxilla of Istieus bears more teeth and the single ovoid supramaxilla is rela- 

 tively larger. The lateral surface of the premaxilla shows one or two small sensory 

 pores. 



Mandible. The lower jaw of Istieus is relatively longer than that of Pterothrissus 

 and results in the mouth being sub-terminal instead of decidedly inferior. Other 

 differences between the lower jaw of Istieus and Pterothrissus include : the position 

 of the coronoid process which is situated posteriorly in Istieus, the relatively smaller 

 size of the articular, and the size of the dentary teeth. In Pterothrissus the dentary 

 teeth are needle-like and of the same size as those on the premaxilla. Those of 

 Istieus are conical (it is admitted that the difference between conical and needle-like 

 is relative rather than absolute), larger and more robust than the premaxillary teeth 

 (Text-fig. 71). 



Further features to be noted concerning the lower jaw of Istieus are the sites of 

 ligament insertion. Immediately anterior to the articulatory cup the posterior 

 margin of the articular is thickened for the insertion of the maxillary-mandibular 

 ligament. Finally, the posterior face of the articular (that area which may be termed 

 the retroarticular process, without the implication that a separate ossification is 

 present) is pitted to receive the interopercular-mandibular ligament. 



Circumorbital series. This series is poorly known. The condition of preservation 

 makes it difficult to identify a supraorbital and the rostral ossicles, of which there 

 were probably three, have become so displaced as to render interpretation of their 

 spatial relationships impossible. The antorbital is a narrow splint-like element. 



The infraorbital series is shown in Text-fig. 71. Although the suture lines between 

 the separate elements are not distinct on any specimen an overall resemblance with 

 Pterothrissus is evident. Certain minor differences occur : the first infraorbital of 

 Istieus grandis is relatively smaller, less rounded and does not bear a dorsal promi- 

 nence ; posteriorly, at the ' elbow ' of the series, the margin is not produced in a 

 process and the sensory canal appears to have lain entirely superficial to the fourth 

 and fifth infraorbitals. These minor differences are overshadowed by the strong 

 points of similarity such as the small dermosphenotic, which is more closely asso- 

 ciated with the skull roof than with the other infraorbitals, the narrow posterior 

 infraorbitals and the large rolled margin. 



Opercular series. A comparison of Text-figs. 71 and 66 shows a close similarity 

 between the constituent elements in Istieus and Pterothrissus. Two minor differ- 

 ences are apparent : the operculum of Istieus is smaller and the postero-ventral 

 part of the preoperculum is less expanded. The anterior limb of the suboperculum 

 of Istieus is smaller than in Pterothrissus. 



