86 OLD RED SANDSTONE FISHES. 



The ventral surface (woodcut, Fig. 47) shows only one articular surface (s.), that 

 at the antero-external angle, which is overlapped by the anterior ventro-lateral 

 plate. On its deep surface, however, we find two along the inner border ; 

 one anterior (woodcut, Fig. 48, t.), which overlaps the median ventral, and 

 another longer one (u.) behind it, which fits over the plate of the opposite side. 



The ascending lamina (PI. XVII, fig. 6 in impression) is joined at nearly a 

 right angle to the anterior three-fourths of the outer margin of the ventral portion 

 of the plate. In front it is very low, but gradually rises posteriorly, until shortly 

 before its termination its height is about equal to half the breadth of the ventral 

 lamina ; it then falls pretty suddenly away in the manner shown in the figure. 

 This vertical lamina shows no articular area on its outer aspect, except a part of 

 that one in front (s.) which is covered by the anterior ventro-lateral ; but on its 

 deep surface there is a long and narrow one along the upper margin, which 

 tapers to a point in front, and covers the corresponding surface on the outside of 

 the lower margin of the posterior dorso-lateral. 



The median ventral plate (PI. XVI, fig. 5, and PI. XVII, fig. 2) is ovate 

 lozenge-shaped, and shows on its ventral aspect four narrow articular arese, two 

 antero-external and two postero-external, which are confluent round the four 

 angles, anterior, posterior, and right and left external, of the enclosed free central 

 space. These areas are overlapped by the corresponding right and left anterior 

 and posterior ventro-lateral plates. 



The articulations of these body-plates are absolutely constant, and they are so 

 complicated that it may be as well to recapitulate them briefly. 



The anterior median dorsal overlaps the anterior and posterior dorso-laterals, 

 and is overlapped by the posterior median dorsal. 



The posterior median dorsal overlaps the anterior median dorsal and the 

 posterior dorso-laterals, but is itself overlapped by none. 



The anterior dorso-lateral overlaps the posterior dorso-lateral, and is over- 

 lapped by the anterior median dorsal and the anterior ventro-lateral. 



The posterior dorso-lateral overlaps no other plate, but is itself overlapped by 

 the anterior and posterior median dorsals, by the anterior dorso-lateral, and by 

 the anterior and posterior ventro -laterals. 



The anterior ventro-lateral overlaps the anterior and posterior dorso-laterals, 

 the posterior ventro-lateral, and the median ventral. The right plate overlaps, 

 and the left one is overlapped by, its fellow of the opposite side. 



The posterior ventro-lateral overlaps the median ventral and the posterior 

 dorso-lateral, and is overlapped by the anterior ventro-lateral. The left plate 

 overlaps, and the right one is overlapped by, its fellow of the opposite side. 



The median ventral overlaps no other plate, but is overlapped by the two pairs 

 of ventrolaterals. 



