84 PLEURACANTHID SHARKS 
essential to sharks. That it was actually a shark cannot 
be doubted; its gills, six or seven in number, opened 
separately to the surface; its teeth (Fig. 90 A) were 
typically shark-like, arranged in many rows on Meckelian” 
and palatoquadrate cartilages ; a tuberculated dorsal spine 
was present ; claspers occurred in the male; the vertebral 
column, although notecherdal, NV, presented intercalary 
plates, /C, and the 
jaw was essentially 
JS & a a hyostylic, HZ. On 
the other hand, 
Fig. 90A.— Teeth of Pleuracanthus. *%# many of its struct- 
(After DAVIS.) 
ures are clearly tran- 
sitional to the Dip- 
noan : the pelvic fins 
are shark-like, with - 
the radial supports, 
R, arising from but 
one side of the line 
of basals, B; but the 
pectoral fin is typi- 
cally archipterygial, 
and the caudal diphy- 
cercal, as inthelung- | 
Fig. 90 B. — Dermal bones of the head roof of fishes. In this r 
Pleuracanthus, X 4. (After DAVIS.) pin eb 
gard the continuous 
dorsal fin, with its separate basals and radials, B and R, is 
again noteworthy. But most singular of all the features 
of this lung-fish-like shark were its integumentary charac- 
ters ; shagreen tubercles had disappeared on the body sur- 
face, and derm bones had appeared roofing the head : their 
arrangement (Fig. 90 &) is strikingly similar to that of the 
lung-fish of Fig. 124. 
