84 OLD RED SANDSTONE FISHES. 



commencement of the posterior fourth of the entire length it descends more 

 suddenly to the posterior angle, its height at the place where it falls away being 

 nearly equal to one-half of the breadth of the ventral lamina of the plate. On the 

 outside aspect there are no area? for articulation of other body-plates, but near the 

 front, about the junction of the first and second fourths of the length of the plate, 

 is found the brachial fossa, with its foramen (jr., fig. 39) and its cup-like brachial or 

 articular process. The latter unfortunately cannot be figured in the present species, 

 as it is impossible to free it from the matrix. Broken surfaces, however, indicate 

 that it was configured as in the Russian A. ornata. 



The deep surface of the horizontal lamina shown in impression in PI. XVII, 

 fig. 3, and restored in woodcut, Fig. 46, shows two blunt ridges, which diverge at an 

 acute angle from a point just in front of the brachial foramen, and of these one, the 

 feebler of the two, passes transversely across to meet its fellow on the bone of the 

 other side ; the other, rather more strongly marked, passes inwards and forwards, 

 and ends on the angle between the inner and anterior margins. On the very oblique 

 posterior margin of the plate is seen first an area (woodcut, Fig. 46, q.) which over- 

 laps the median ventral ; behind and external to which, and also passing somewhat 

 round on to the vertical lamina, is another (r.), overlapping the posterior ventral 

 lateral. In addition to these the plate of the right side shows a narrow area Q>.) 

 along the inner margin, which overlaps the corresponding margin of its fellow 

 of the opposite side. 



On its internal surface the vertical lamina shows in front the brachial perfora- 

 tion (fr.), and along its upper margin a narrow articular band for the anterior 

 dorso-lateral plate, this commencing just above the situation of the foramen. The 

 oblique posterior margin of this lamina shows also a tolerably broad articular area, 

 overlapping the posterior dorso-lateral plate, and also a small part of that which 

 overlaps the posterior ventro-lateral. Those details are seen in impression on the 

 natural cast represented in PI. XVII, fig. 4. 



The semilunar plates (s.L, woodcuts, Fig. 45 and 46) form together the middle 

 part of the anterior margin of the ventral wall of the carapace. Each of them is 

 small, narrow, and transversely elongated, with a slightly concave free anterior 

 margin, a short inner one apposed to its fellow of the opposite side, while the 

 posterior and outer margins, in contact with the anterior ventro-lateral plate, pass 

 with a curve uninterruptedly into each other. They thus fill up the corresponding 

 narrow space cut out from the inner half of the anterior margin of the ventro- 

 lateral. 



The posterior ventro-lateral plate, like the anterior, takes part in the formation 

 both of the ventral and lateral walls of the carapace, and its ventral lamina 

 (woodcuts, Figs. 47 — 50) is also unsymmetrical on the two sides of the body, 

 only in this case it is the left plate which overlaps the right. The ventral lamina 



