i 4 2 ELOPIFORM FISHES 



Material. Seventeen specimens in the B.M.N.H. from the Campanian of Senden- 

 horst, Westphalia, Germany. The fish are mainly preserved in impression in a 

 fine-grained, grey limestone. The bone, where present, is soft and poorly preserved. 

 In studying the material rubber casts were used. 



Remarks. In a great many features there is close agreement between the anatomy 

 of Pterothrissus and Istieus. This similarity necessitates only a brief comparative 

 description of Istieus grandis. 



Description. Neurocranium. The neurocranium, like that of Pterothrissus, is 

 long and shallow. The snout is less elongated than that of Pterothrissus and does 

 not exhibit the sharp anterior curvature. In dorsal view (Text-fig. 70) several 

 minor differences from Pterothrissus (Text-fig. 60) are evident ; the dermethmoid 

 and pterotic of Istieus are relatively smaller, the parietal is only marginally longer 

 than broad, and the frontal is wider, particularly above the orbit. As in Ptero- 

 thrissus, the dermethmoid bears a prominent median ridge supported anteriorly by 

 inclined struts, the lateral margin of the frontal is raised into a ' mudguard ' above 

 the eye and the posterior margin of the pterotic shows a narrow rebate for the 

 supratemporal commissure. In Istieus the nasal is represented by a long gutter-like 

 element, there being no evidence of two separate ossicles as in Pterothrissus. 



A lack of detail in the snouts of all the specimens leads one to suppose that, as in 

 Pterothrissus, much of the ethmoid was cartilaginous. The lateral ethmoid of Istieus 

 is the same shape as that of Pterothrissus but relatively larger. 



Little is known about the lateral wall of the otic region. B.M.N.H. P. 3885a 

 shows an internal view of the right side, although little bone remains. This specimen 

 shows that the bulla which contained the sacculith is large, the subtemporal fossa is 

 deep and there is a deep depression posterior to the ascending wing of the para- 

 spenoid. In all that may be seen the lateral wall of the braincase is similar to that 

 of Pterothrissus. 



The parasphenoid, as in Pterothrissus, is straight and narrow and runs from beneath 

 the lateral ethmoid to beneath the middle of the otic region. Woodward (1901) 

 stated that no parasphenoid teeth could be seen, but B.M.N.H. P. 3885a clearly 

 shows their presence. There are many more teeth on the parasphenoid than in 

 Pterothrissus. Each tooth is conical with a rounded tip. The vomer is narrow and 

 edentulous but does not show the semicircular median ridge seen in Pterothrissus. 



Hyopalatine bones. The hyopalatine series, as in all pterothrissids, is longer than 

 deep and the quadrate/mandibular articulation lies beneath the posterior half of the 

 orbit. 



The shape of the hyomandibular is very similar to that of Pterothrissus but the 

 head is formed of thicker bone. Antero-ventrally the hyomandibular is partially 

 overlapped by the metapterygoid, a feature not seen in Pterothrissus. However, 

 the hyomandibular-metapterygoid foramen is developed in typical fashion. As in 

 Pterothrissus the inner surface of the metapterygoid is grooved for the reception of 

 the flattened symplectic. 



The endopterygoid is relatively large and bears more teeth than in Pterothrissus. 

 The teeth all appear to be conical but they probably varied considerably in size as 

 witnessed by the irregular size of the alveoli in B.M.N.H. 35012. The topographical 



