FAUNA OF THE GOSAU FORMATION. 



653 



one which measures nearly 5 inches in length has a subrhomboidal 

 outline, two long sides converging in front, and a short pair of sides 

 converging behind. The greatest width of the plate is 2g inches. 

 The greatest length of the flat part of the base is 3| inches ; and 

 the posterior 1| inch rises into a strong spine, which terminates the 

 median crest, is 1 T 9 ^ inch high, compressed behind and above. The 

 crest gradually diminishes in height from this spine forward till it 

 dies away at the anterior end. The outline of the crest is very 

 slightly sigmoid. The crest has a compressed aspect, as though it 

 had been naturally squeezed from side to side in its upper half. 



There are numerous smaller sharply carinate plates of a some- 

 what ovate outline, with the keel placed nearer towards one margin 

 than the other, and always becoming a little more elevated towards 

 one end, where it is truncated. And these plates, though mostly flat 

 on the underside, always have the end on which the ridge is highest 

 bent a little upward, as though to overlap the next succeeding plate. 

 These plates vary in size : one is 2^ inches long, 1 T 6 ^- inch wide, 

 and has the keel ^-J inch high ; another is 2-^ inches long, l T 7 ff inch 

 wide, and has the keel ^ inch high posteriorly. 



Another remarkable series of plates is distinguished by extreme 

 thinness. They appear all to have been subrhomboidal and to have 

 had the keel scarcely elevated. 



The largest is about 2y^- inches long, and more than 2 T \ inches 

 wide. The under surface is smooth and slightly convex. The su- 

 perior and inferior margins converge to a sharp but irregular edge ; 

 the thickness of the body of the plate is about inch, though many 

 of the plates are much thinner ; and the thickness in the line of the 

 median ridge is abont j% inch. This slight keel does not extend to 

 either extremity of the plate: but the margin of each plate is turned 

 up towards one of the posterior sides, as though they still obliquely 

 overlapped (PI. XXXI. fig. 3). The surface of these plates is 

 slightly concave on each side of the median ridge, and there scored 

 with vascular markings which ascend towards the ridge and ramify 

 and interlace. Their prevailing direction in the plates with more 

 elevated keels is towards the posterior spine. There are a few 

 slightly thicker plates which have no trace of keel, but are flat 

 below and gently convex above (Pi. XXVIII. fig. 5), with a deep 

 Y-shaped vascular groove on each, and a sharp margin. The 

 smallest and best-preserved is 1 T 4 ^- inch long, and inch wide. 



Some fragments of crest-spines, broken away from the bases, in- 

 dicate plates of a larger size than any thing here described. The 

 plates appear to have been remarkable for their great side-to-side com- 

 pression, the posterior elevation of the crest, and the sharpness of 

 the spine, which, in fragments preserved, extended to a height of 5 

 inches where the antero-posterior measurement is only about 3 

 inches, and the greatest thickness of the spine from side to side is only 

 inch (at the inferior fracture). 



There is also a fragment indicating that the plates in which the keel 

 is almost suppressed, in some regions attained a larger size than has 

 here been described. 



