FOSSIL AND RECENT 141 



prominence, differ little from the other body scales. The position of the nucleus of 

 the scale varies slightly according to the position of the scale on the body. Anterior 

 scales show an anteriorly situated nucleus. The converse is true of those scales 

 situated posteriorly. Five to eight short radii are found within the anterior field 

 of the scale but they do not reach the nucleus. Between the radii the anterior edge 

 of the scale is slightly scalloped. Radii are less numerous on lateral line scales and 

 the margins are generally smoother. The dorsal, ventral and anterior fields show 

 well-marked circuli. In the anterior field the circuli tend to break up to form 

 numerous backwardly pointing tubercles. All scales have enclosed bone cells 

 which are generally more numerous towards the edges of the anterior, dorsal and 

 ventral fields. 



Genus ISTIEUS Agassiz, 1844 



1844 Istieus Agassiz : 5, pt. 2, p. 91. 



Diagnosis. Pterothrissid fish in which the interorbital distance is equal to one- 

 third of the length of the neurocranium. Parasphenoid tooth patch extending from 

 beneath the lateral ethmoid to the level of the parasphenoid ascending wings. 

 Maxilla bearing many teeth. Mandible with a weakly developed coronoid process 

 situated in the posterior third of the jaw. Dentary teeth larger than those of the 

 upper jaw. 



Type-species. Istieus grandis Agassiz. 



Istieus grandis Agassiz 

 (Text-figs. 70-73) 



1833-44 Istieus grandis Agassiz : 5, pt 1, p. 13 ; pt 2, p. 92, pi. 18. 



1833-44 Istieus microcephalus Agassiz : 5, pt 1, p. 13 ; pt 2, p. 94, pi. 17. 



1833-44 Istieus macrocephalus Agassiz : 5, pt 1, p. 13 ; pt 2, p. 94, pi. 16 (lower figure) (errore). 



1858 Istieus grandis Agassiz ; Marck : 246. 



1858 Istieus microcephalus Agassiz ; Marck : 246. 



1863 Istieus macrocoelius Marck : 37, pi. 4, figs. 1-5. 



1863 Istieus mesospondylus Marck : 38, pi. 5, fig. 1. 



1873 Istieus macrocoelius Marck ; Marck : 59, pi. 2, fig. 2. 



1885 Istieus macrocoelius Marck ; Marck : 253. 



1901 Istieus grandis Agassiz ; Woodward : 67. 



1954 Istieus grandis Agassiz ; Siegfried : n, pi. 2, figs. 5, 6. 



Diagnosis. Istieus reaching 550 mm S.L. Length of head 30 per cent S.L., 

 maximum depth of body approximately 22 per cent S.L. Operculum one-and-a-half 

 times as deep as wide. 8-10 branchiostegal rays. Vertebrae 87-92, dorsal fin-rays 

 53-57- 



Holotype. Imperfect fish, Museum National d'Histoire Naturelle, Paris, from 

 the Campanian of Sendenhorst, Westphalia, Germany. 



