88 OLD RED SANDSTONE FISHES. 



external marginal of the forearm; its dorsal and ventral reflected margins are 

 each divided into two by an obtuse angle which is nearer the shoulder on the 

 ventral than on the dorsal side, and above these angles on each side re- 

 spectively the corresponding articular and anconeal plates are fitted. 



The internal marginal (i. m.) is shorter than the external one. It is shaped 

 somewhat like it, being narrow above and below and longitudinally folded on 

 itself, but the line of the fold forms the dorso-internal margin of the limb, so that 

 this plate does not appear at all on the ventral side, and only in internal casts 

 (PI. XVIII, fig. 3) can its impression be obliquely seen, when the cast is looked at 

 from the ventral aspect. The two parts into which it is divided by the longitudinal 

 fold are likewise unsymmetrical, as that seen on the dorsal surface of the limb is 

 broader than the other which is placed on the inner aspect. The upper extremity 

 of this plate is pointed ; its lower one fits over the articular process of the upper 

 internal marginal of the forearm ; its convexly angulated dorsal margin articu- 

 lates with the dorsal articular and the dorsal anconeal ; while the corresponding 

 ventral margin joins in like manner with the ventral articular and the ventral 

 anconeal. 



The dorsal anconeal (d. a.) is irregularly triangular, the base of the triangle 

 being at the elbow-joint, while the truncated apex touches the distally directed 

 and similarly truncated apex of the dorsal articular. The slightly convex outer 

 margin is in contact with the external marginal. The inner, which is rather con- 

 cave, articulates with the internal marginal ; the lower, convex, and showing ex- 

 ternally a narrow, smooth, articular surface, passes over the upper extremity of 

 the corresponding upper central of the forearm, below which it is lodged in a slit 

 on the outer surface. 



The ventral anconeal (v. a.) is slightly different in shape. It is longer than the 

 dorsal one, pointed above, and its surface is unequally divided by an angular longi- 

 tudinal fold, the broader portion being ventral, the narrower internal in position, 

 and in this way the ventral anconeal forms the inner margin of the ventral aspect 

 of the limb below the ventral articular, and to the exclusion of the internal 

 marginal. Its outer margin articulates with the ventral articular proximally, 

 but for the greater part of its length with the external marginal ; its inner margin 

 articulates with the internal articular and internal marginal ; and the part which 

 its lower margin takes in the structure of the elbow-joint is similar to that 

 described above in the case of the dorsal anconeal. 



The Forearm. — The lower arm or forearm is flattened from above downwards, 

 having two acute edges, external and internal ; it is curved so that the outer edge 

 is convex, the inner somewhat concave ; finally it is sharply pointed. Its structure 

 is simple, as may be seen in PI. XVIII, fig. 7. 



There are two central plates (c), dorsal and ventral. Each of these is elongate 



