PLEURACANTHUS , 
f. 6% 
w 
to have been present, 
and suggest strongly 
continuous fin-fold char- 
acters. (V. p. 40.) 
Pleuracanthus (Fig. 
90), the third of the 
well-known Palzozoic 
sharks, is widely differ- 
ent from the Acantho- 
dian: it suggests a tran- 
sitional form between 
the generalized Cladose- 
lachian, on the one hand, 
and the Dipnoan on the 
other; or, more accu- 
rately, it demonstrates 
that the stems of shark 
and lung-fish were at one 
time drawn very closely 
together. It has thus 
far occurred only in the 
Carbon and Permian, 
but may reasonably be 
expected in lower hori- 
zons as a contemporary 
of the earliest lung- 
fishes. 
Pleuracanthus is in 
many ways the most in- 
teresting and suggestive 
member of the shark 
group; for it destroys - 
many of our conventional ideas as to the general characters 
HM. Hyo- 
HA, Heemal arches. 
N. Notochord. MA, Neural process and spine. 
R'. Rib. SG, Shoulder girdle. 
DS. Dermal head spine. 
MC. Mandible (Meckel’s cartilage). 
(Restoration slightly modified after A. FRITSCH.) From the 
xX about }. 
FR. Radial fin cartilages. 
PPS ee 
erdenu es ote 
A 
PQ. Palatoquadrate, 
MARS 
Meade cr, 
MAD aI 
4, Basal fin cartilages. D. Dermal margin of fin. 
/C. Interneural plates. 
LLL fy tp PP, 
pita 
SIP yy 
<LS72 
Fig. 90.— Pleuracanthus decheni (Goldf.), 2. 
Permian of Bohemia. 
A’. Anal fin, 
mandibular. 
Pelvic cartilage (girdle), 
