POPULAR SCIENCE REVIEW. 
6 
animal of which traces have been met with, is represented only 
by a very few imperfect specimens. It was described by the 
late Mr. J. W. Salter, F.G.S., in 1859 (“Annals and Mag. Nat. 
Hist.,” Vol. IV., p. 45), and more fully in Professor Lankester’s 
Monograph on the “ Fishes of the Old Red Sandstone ” (Part I., 
1867, p. 25, part II., fig. 4, “Mon. Pal. Soc.,” Vol. XXI.). 
It belongs to the Pteraspidean di- 
vision of fish-shields (for which the 
term Hetevostraci f has been proposed 
by Professor Lankester), and to that 
simplest type the genus Scaphaspis 
(Woodcut, Fig. 3), in which the shield 
is of an oval form, and composed of a 
single piece about one inch and a half 
in length, resembling a much elon- 
gated ellipse, truncated at both ends. 
There is a small spine, or tubercle, 
near the posterior margin of the 
shield, the outer surface of which is 
marked by coarse longitudinal striae. 
J/iAUU.'i.U KJJC inn omm 1/ \'a 1/ O 
Scaphaspis (after Lankester). It is a very remarkable fact that not 
r. rostrum, or rostral region. a trace of “ osseous lacunae ” have 
</. central disc, or discal region, been detected in the heterostra- 
s. posterior spine. cous } iea( j. s bields. Externally, the 
surface, when well preserved, is covered with minute grooves and 
ridges, running parallel to, and concentrically with, one another. 
These surface-markings (Pl. I., Fig. 1, a) may not unfittingly 
be compared to the markings of the epidermis on the palms of 
the hands and fingers in man and other Primates. The inner 
surface of the shield is quite smooth and polished, and exhibits no 
Vascular channels, as in Cephalaspis, only a few irregular ridges 
diverging from the centre of the shield, or running parallel to 
the margin. 
Professor Huxley has minutely described and illustrated the 
microscopic structure of this oldest-known type of FishesJ The 
shield is composed of three layers, the outer or “ striated ” 
layer (PI. I., Figs. 1 and 2, a, a), the middle or “ cancellated ” 
(if. 5, 6), the inner or “ nacreous” layer (ib. c, c). This inner 
nacreous layer is made up of laminse of about -^gV^th of an 
inch, arranged horizontally, and is totally devoid of vascular 
# For the use of this and nine other illustrations of the genera of Cephal-* 
aspidean Fishes in this paper, the author is indebted to the kindness of the 
Kev. T. Wiltshire, Sec. G. S., Hon. Sec. Pal. Soc. ; they illustrate ProL 
I>ankester’s Monograph in the Vols. of the Pal. Soc. for 1867 and 1869. 
t From erepos of another kind, and oarpaKov a shell or dermal bone. 
^ “ Quart. Journ. Geol. Soc." 1858, Vol. xiv. p. 267. Plates xiv. and xv* 
