22 
POPULAR SCIENCE REVIEW. 
Thus we should have the following modifications in Pisces*: — 
Notochordal — destitute of endo- or exoskeleton ; respi- 
ratory and alimentary system rudimentary — resem- 
bling the Tunicata. No fossil type known. 
(First fishes actually known). 
Cephalaspidce. 
Placodermi. 
Chondrostei. 
Qi'ossopterygidce. 
Lepidosteidce. 
Plagiostomi or 
Elasmobranchii. 
Notochordal — endoskeleton absent (or rudimentary ?) ; 
exoskeleton in the form of bony bead-shield and, 
in some, dermal scutes. 
f Notochordal — peripheral processes of endoskeleton 
4 (and fin-rays) ossified ; exoskeleton well developed 
1^ anteriorly, both on dorsal and ventral surface, 
f Notochordal — peripheral endoskeleton cartilaginous, 
4 rudimentary ; head covered by dermal plates — scutes 
( in rows on body. 
Notochordal — peripheral endoskeleton partly ossified; 
dermo-cranial bones, coated with enamel; scales 
rhomboidal or cycloidal ; fins lobate, paired, central 
axis of fin covered with scales ; air-bladder double- 
celled, in some the walls are ossified. 
f Vertebral skeleton, perfectly ossified ; exoskeleton a 
\ complete suit of enamelled bony plates and scales. 
{ Vertebrce — partly ossified , partly cartilaginous ; endo- 
skeleton chiefly cartilaginous ; skin roughened with 
shagreen, tubercles, and spines. 
( Incoming of thin-scaled fishes). 
Caturidce. 
Notochordal — neural and haemal processes well de- 
veloped ; head with ganoid covering ; body covered 
with thin scales. 
(Modern fishes). 
Tdeostei. 
f Endoskeleton well ossified ; dermal covering con- 
[ sisting of thin u cycloid,” or u ctenoid,” scales. 
(Transitional forms, or possibly Amphibia?) 
f Notochordal — neural and haemal processes ossified; 
Protopteri or J only two pectoral and two ventral fins; dermal 
Dipnoi. | covering of cycloidal scales; functional internal 
[ branchiae and well developed internal lungs. 
EXPLANATION OF PLATE I. 
Fig. 1 . A portion of the test of Scaphaspis Lloydii ; considerably enlarged, 
to show the form of the surface-ridges and the coarser struc- 
ture of the lower layers of the shield. The piece is from near 
the centre of the shield. Observe the flatness of the ridges 
a = the outer, or ‘ striated 1 layer ; b = the middle, or ‘ can- 
cellated ’ layer ; c = the inner, or 1 nacreous ’ layer (after 
Lankester). 
* These cannot be deemed to be placed in their proper classificatory order, 
but only to show the gradations of skeletal development. 
