Structure and Classification of the Asterolepidte. 491 



overlapping plate being thinned off to fit on to a corresponding 

 band along the margin of the outer surface of the one over- 

 lapped. The hexagonal anterior dorsal plate {a. d.) in this 

 way overlaps the anterior dorso-laterals, but is itself over- 

 lapped along its postero-lateral margins by the posterior 

 dorso- laterals, and also behind by the posterior median dorsal, 

 though in this latter case the contrary is stated by Hugh 

 Miller (i"6. p. 309). 



The anterior dorso-lateral {a. d. I.) overlaps the posterior 

 dorso-lateral, but is itself overlapped by the anterior median 

 dorsal and by the anterior ventro-lateral. 



The posterior dorso-lateral {p. d. I.) overlaps the anterior 

 median dorsal, but is itself overlapped by all the other plates 

 with which it is in contact, viz. the posterior median dorsal, 

 the anterior dorso-lateral, and the anterior and posterior 

 ventro-laterals. 



The anterior ventro-lateral overlaps the anterior and poste- 

 rior 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 Astero- 

 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 transverse joint into two seg- 

 ments, proximal and distal, rather flattened above and below, 

 especially towards the extremities, and composed of numerous 

 plates, which have much the same contour above and below. 

 In the proximal segment (PI. XVII. figs. 1 and 2) we have 

 the following plates :— two articular (a/-), 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 mar- 

 ginal, and two anconeal, or elbow-pieces {a), dorsal and ventral, 

 somewhat triangular in shape, their apices directed forwards 

 to meet the posterior extremities of the articulars, their con- 

 vexly 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 shajie, 

 with the acute angles truncated, one acutely pointed terminal 

 {t), and four marginals {in), of which two follow each other 



