FOSSIL AND RECENT 151 



Istieus gracilis, so named because of the apparent slender form of this species, was 

 considered by Woodward (1901) to represent specimens of I. grandis which had 

 become distorted during preservation. I have examined neither the holotype nor 

 many specimens which have been referred to this species. Two examples preserved 

 in a single slab (B.M.N.H. P.3889), which were compared by Woodward (1901) to 

 forms described as /. gracilis, show meristic counts similar to those of /. grandis. 

 The evidence available to the author is not sufficient either to include the ' /. gracilis ' 

 form with the type-species or to recognize a distinct species. Siegfried (1954 : 11) 

 recognizes /. gracilis on the basis of the slender body form. 



Istieus lebanonensis is known only from the holotype, which is poorly preserved. 

 Little may usefully be added to the descriptions given by Davis (1887) and Woodward 

 (1901). The size and dorsal fin-ray count are closer to those of /. macrocephalus 

 than /. grandis. 



Affinities of the genus Istieus 



Istieus was placed by Agassiz (1833-44) m the family Esocoides as the only 

 marine genus in an otherwise freshwater group of fishes. In establishing the genus 

 Agassiz (op. cit.) recognized four species, /. grandis, I. macrocephalus, I. gracilis and 

 I. microcephalus. With the exception of /. macrocephalus the descriptions and figures 

 given by Agassiz do not justify separation at the species level. 



Marck (1863) followed Agassiz, both with regard to familial placement and the 

 recognition of several species, but he later (1873) combined /. grandis and /. micro- 

 cephalus in a single species to which he gave the name /. macrocoelius. Such an 

 action is contrary to the rules of nomenclature. Marck (1863) also established a new 

 species, /. mesospondylus , which is here considered a synonym of the type-species. 



Woodward (1901) recognized two species from Westphalia (/. grandis and I. 

 macrocephalus) and another from Sahel Alma (/. lebanonensis). These are the only 

 species recognized here. Woodward (1901) placed the genus in the Albulidae to 

 which it is clearly related. 



Istieus is very similar to the extant Pterothrissus, a view expressed by Woodward 

 (1901) who stated that the two genera are not easily distinguished from one another. 

 This similarity is endorsed by Greenwood et al. (1966) and Goody (1969b). Differ- 

 ences that do exist between the genera are minor. Many have been mentioned 

 above in the description of Istieus grandis. The more important differences are 

 mainly in degree, not absolute : thus Istieus shows a larger head ; a wider skull 

 roof ; a more terminal mouth ; a smaller first infraorbital which reaches well for- 

 ward ; more extensive parasphenoid teeth ; more teeth on the endopterygoid and 

 (particularly) the ectopterygoid ; and finally more teeth on the maxilla. In all 

 these respects Istieus may be considered more primitive than Pterothrissus. Absolute 

 differences between the genera are few. Istieus shows a difference in tooth size 

 between those teeth borne by the upper and lower jaws, a well-developed ridge 

 upon the metapterygoid, two epurals and a posteriorly situated coronoid process. 

 In contrast Pterothrissus has premaxillary and dentary teeth of equal size, no well- 

 defined ridge on the metapterygoid, three epurals and an anteriorly situated coronoid 

 process. 



