FOSSIL AND RECENT 149 



Holotype. B.M.N.H. P.3892, a nearly complete fish exhibiting a dorso-ventrally 

 crushed cranium and lacking part of the tail. From the Campanian of Baumberg, 

 Westphalia, Germany. 



Material. The holotype and B.M.N.H. 1275, a complete fish from the same 

 locality as the holotype. The specimens are preserved in a buff-coloured limestone. 



Remarks. Agassiz (1835) separated /. macrocephalns from other species of the 

 genus on the basis of its larger head. This difference is more apparent than real. 

 The head length in the type-species and /. macrocephalus is similar, 30-34 per cent 

 of the standard length. The maximum depth of the head is also comparable. Marck 

 (1858, 1863) also recognized a species I. macrocephalus and followed Agassiz in quoting 

 the size of the head as the diagnostic character, but parenthetically it should be added 

 that the figure of I. macrocephalus given by Marck (1863 : pi. 4, fig. 6) shows a dorsal 

 fin-ray count unlike that of Agassiz's type specimen. Thus, /. macrocephalus 

 described by Marck may not be that of Agassiz (1835). 



Woodward (1901 : 71) recognized /. macrocephalus as a good species, but made no 

 mention of head length. Instead he separated I. macrocephalus from the type-species 

 on the basis that it had fewer dorsal and anal fin-rays, that the anal fin arises behind 

 the dorsal and that the whole fish is of a more slender form. 



The dorsal fin-ray count is decidedly lower (41-44 in /. macrocephalus, 53-57 in 

 /. grandis) and the fish is more slender, a feature probably giving the mistaken 

 impression of a larger head. In /. macrocephalus the body tapers more markedly 

 to a narrower caudal peduncle than in I. grandis. Contrary to Woodward's opinion 

 (1901 : 71) the anal fin actually arises in the same position relative to the dorsal fin 

 and vertebrae in both species, as may be seen by a comparison of Text-figs. 73 and 

 74. The apparent posterior insertion of the anal fin in the type specimen of /. 

 macrocephalus is due to the absence of the posterior dorsal fin-rays in the fossil. 



Other differences include : the vertebral count, 87-92 in /. grandis, not more 

 than 75 in I. macrocephalus ; the operculum of /. macrocephalus is narrower (depth : 

 width ratio of this bone is 2 : 1 against 1-4:1 in the type-species) and has a more 

 oblique ventral margin ; there are only 6 branchiostegals in /. macrocephalus against 

 8-10 in I. grandis. Finally, the snout appears more rounded in /. macrocephalus, 

 but this may be due to distortion during preservation. 



Thus, /. macrocephalus may be separated from the type-species on the shape of the 

 operculum, the number of branchiostegal rays, dorsal fin-ray and vertebral counts 

 and the caudal peduncle depth. The general trend in Pterothrissidae appears to 

 be towards elongation of both the vertebral column and dorsal fin. In this respect 

 /. macrocephalus is more primitive than /. grandis ; in contrast, the lower number of 

 branchiostegal rays would suggest that /. macrocephalus is more specialized than I. 

 grandis. 



Remarks on other species 



Two other species of Istieus have been described, I. gracilis Agassiz from the 

 Campanian of Sendenhorst and I. lebanonensis Davis from the Upper Santonian of 

 Sahel Alma in the Lebanon. 



