STENEOSAUEUS. 81 



looking almost directly upwards ; a small preorbital opening usually present. Frontal 

 small and the skull-roof much flattened, passing quite gradually into the upper surface 

 of the snout. The alveolar border straight, without undulations. Internal narial 

 opening rounded, palatal vacuities of moderate size. Teeth numerous, with the 

 enamel marked by longitudinal ridges, of which, as a rule, one on the anterior and one 

 on the posterior side of the crown, at least near the point, form well-marked carina?. 



Vertebra? slightly concave, the posterior caudals all with backwardly directed spines, 

 there being no sharp deflection of the end of the tail, such as occurs in the Metrio- 

 rhynchida?. Fore limb not reduced to a paddle-like structure as in the above- 

 mentioned family. The tibia about half the length of the femur, and the sum of the 

 length of the femur and tibia considerably more than twice the sum of the lengths of 

 the humerus and radius. Armour consisting of a double row of keeled plates running 

 down the mid-dorsal line, the successive plates uniting by an overlapping and peg-and- 



Text-fig. 31. 



imx. 



Semi-diagrammatic figure of the upper surface of the skull of Steneosaurus durdbrivensis. 



(About ^ nat. size.) 



hoc, basioccipital ; /., frontal ; /., jugal ; /., lachrymal ; m.v., maxilla ; n., nasal ; nar., external nares ; 

 par., parietal ; pmx., premaxilla ; po.f., postfrontal ; pr.f., prefrontal ; jit., pterygoid ; q., quadrate ; 

 q.j., quadrato-jugal ; sq., squamosal. 



socket articulation; also numerous other plates, probably ventral, the arrangement 

 of which is uncertain. 



Middle and Upper Jurassic. 



Mystriosaurus of Kaup (in Bronn's Lethsea Geogn. ed. 1, vol. i. 1837, p. 525) is by 

 many writers regarded as a synonym of Steneosaurus, but it should be used for the 

 Liassic forms, which, though closely similar and probably ancestral to Steneosaurus, are 

 distinguished by the form of the ventral border of the internal nares, which, instead of 

 being a continuous curve, is sharply pointed, the palatines meeting in the middle line 

 in an acute angle ; moreover, the frontals are larger and the temporal fossae relatively 

 smaller. A number of other names given to fragmentary material and insufficiently 



pari n. M 



