Structure and Classification of the Asterolepidio. 31 



the anterior dorso-lateral, and the anterior and posterior 

 ventro-laterals. 



The anterior ventro-lateral overlaps the anterior and pos- 

 terior dorso-laterals, the posterior ventro-lateral, and the 

 median ventral^ while the right one overlaps its fellow of 

 the opposite side in the mesial line. 



The posterior ventro-lateral overlaps the median ventral 

 and the posterior dorso-lateral, but is in turn overlapped by 

 the anterior ventro-lateral. In the middle line the plate of 

 the left side overlaps its fellow. 



The Arms. — These are comparatively short, as in Aster o- 

 lepis, and I find their structure to be essentially similar to 

 those in that genus as described and figured by Pander. 

 They are hollow, divided by a tran verse joint into two 

 segments, proximal and distal, rather flattened above and 

 below, especially towards the extremities, and composed of 

 numerous plates, which have mu.ch the same contour above 

 and below. In the proximal segment (PL L, Figs. 1 and 2) 

 we have the following plates : — two articular (ar), dorsal 

 and ventral, which grasp the helmet-process of the anterior 

 ventro-lateral plate ; one internal articular, only visible from 

 the inner side of the limb, and therefore not shown in the 

 figures; one external marginal (m), extending nearly along 

 the whole of the outer aspect of the segment ; one shorter, 

 internal marginal, and two anco7ieal, or elbow-pieces («), 

 dorsal and ventral, somewhat triangular in shape, their 

 apices directed forwards to meet the posterior extremities of 

 the articulars, their convexly rounded bases articulating with 

 the central plates of the distal segment. The distal segment 

 or " forearm " consists of two centrals (c), dorsal and ventral, 

 rhombic in shape, with the acute angles truncated, one 

 acutely pointed terminal (t), and four marginals {m), of 

 which two follow each other on the outer aspect, and two 

 are similarly placed on the inner aspect of the limb. 



The Tail. — In most Scottish examples of Pterichthys more 

 or less perfect remains occur of a tail, covered with small 

 rounded or somewhat hexagonal, slightly imbricating scales, 

 which are arranoed in lonoitudinal rows and also in trans- 

 verse bands, the scales of one band alternating with those of 



