82 OLD RED SANDSTONE FISHES. 



broad triangular area (z.) which fits on to a similar one on the surface of the 

 posterior ventrolateral. Then we' observe that the anterior margin, which 

 overlaps with a thin edge the posterior border of the cranial shield, sends inwards 

 a sort of ledge, on the anterior surface of which are two diverging ridges or 

 processes (h.), which fit below the external occipital, so that the joint between this 

 plate and the head is of a very complicated character. 



The posterior dorso-lateral (PI. XVI, fig. 4 ; woodcut, Fig. 44) is of an irregularly- 

 oblong shape, and shows six sides, — superior, inferior, postero- superior, postero- 



Fig. 44. 



Fig. 44. — Sketch of the outer aspect of the right posterior dorso-lateral plate. L, articular surface for 

 the posterior median dorsal ; j., for the anterior median dorsal ; k., for the anterior dorso-lateral ; I., for 

 the anterior ventro- lateral ; m., for the posterior ventro-lateral ; n., lateral groove. 



inferior, antero-superior, and anterior. Of these the postero-inferior is free, and 

 forms the lateral boundary of the caudal opening of the carapace. 



The external surface presents a longitudinal lateral line groove (n.), which must 

 be placed horizontally in holding the plate in its natural position ; and five articular 

 surfaces, one along each margin except the postero-inferior, so that this plate, 

 though overlapped by five others, itself overlaps none. The narrow surface (i.) 

 along the postero-superior margin is for the posterior median dorsal plate ; that 

 along the upper margin (j.) receives the corresponding posterior facet of the outer 

 margin of the anterior median dorsal. Above and in front, the third articular 

 space (Ic.) is in the form of an obtuse-angled triangle, whose broad base is formed 

 by the postero-superior margin of the plate, and whose apex is directed backwards ; 

 and this is overlapped by the posterior extremity of the anterior dorso-lateral. A 

 fourth broad articular space (Z.), overlapped by the anterior ventro-lateral plate, 

 corresponds with the antero-inferior margin ; while the fifth (m.) lying along the 

 inferior edge, and covered by the posterior ventro-lateral plate, is narrow, and, 

 though angularly expanded behind, tapers to a point anteriorly. 



The inner surface of the plate is hollowed and smooth, and of course presents 

 no articular arese. Close to the posterior extremity a low, broad, rounded ridge 

 passes nearly vertically upwards, corresponding in position to the angular 



