ASTEROLEPIDm. 
211 
elements, the anterior (a.m.d.) larger than the posterior (p.m.d.) ; 
and these are bounded by two dorso-lateral pairs ( a.d.l . and p.d.l.), 
of which the hinder is much the largest. There are two pairs of 
ventro-lateral plates ( a.v.l . and p.v.l.), which meet in the mesial line 
below, and are sharply reflexed upwards at the sides to overlap the 
inferior edge of the dorso-laterals; while on the ventral surface 
there occurs a small, median, diamond-shaped space between the 
inner truncated angles of these plates, filled by a much-overlapped 
median ventral ( m.v .). The slightly excavated front border of the 
anterior ventro-laterals is filled by a pair of small semilunar plates 
(s.l.) tapering outwardly; and again in advance of these is a pair of 
much larger, transversely elongated elements ( mn .), concave above, 
which have been termed mental by Traquair. The latter plates are 
loosely fixed and often displaced (see PI. V. fig. 3, mn .), but can 
scarcely be interpreted as a mandible. At a point somewhat in 
advance of their hinder extremities, the posterior ventro-lateral 
plates are distinctly constricted, with an inner transverse thickening; 
and this may mark the termination of the abdominal cavity. 
Near their front extremity the anterior ventro-lateral plates are 
strengthened by a robust transverse ridge on the visceral aspect, 
and close to this the pectoral appendages are fixed by a most 
complex, ginglymoid articulation. Each appendage is completely 
encased in closely-fitting plates; and a large orifice in the 
supporting articular facette bears witness to the passage into its 
interior of well-developed vascular canals and nerves. A powerful 
articular plate (<??\), with rounded proximal end, occurs both on tbe 
dorsal and on the ventral aspect of the appendage ; an inner and 
an outer marginal (m.), with an upper and a lower median anconeal 
plate (a.), are closely united with these, and at the distal extremity 
of this group of plates the appendage is jointed. The distal 
segment is shorter and smaller than the proximal, consisting of an 
upper and lower central piece ( c .), a pointed terminal plate (t .), and 
two pairs of marginals (m.). 
The tail is comparatively small, covered with imbricating rounded 
or hexagonal scales, with a series of large azygous ridge-scales on 
the dorsal aspect. The body-scales are thin and finely tuberculated 
(see Nos. P. 3209, P. 4036), while the dorsal ridge-scales are 
comparatively robust. The latter are interrupted shortly behind 
the posterior median dorsal plate by a small triangular dorsal fin ; 
this being membranous, and only stiffened on its front margin by one 
(or perhaps two) of the scales, which might be mistaken for a spine. 
Behind the fin, the ridge-scales are very deeply imbricating to the 
extremity of the tail, which is somewhat upturned (PI. V. fig. 5), 
p 2 
