XVII ELASMOBRANCHII-——-ICHTHYOTOMI 
is acentrous, and the persistent notochord supports 
‘basidorsal cartilages, which alter- 
nate with small interdorsals, a 
series of basiventrals supporting 
small ribs, and in the caudal 
region well-developed haemal 
arches. The dorsal fin is sup- 
ported by slender, tri-segmented 
radialia, which appear to be 
twice as numerous as the neural 
arches in the trunk, but in the 
dorsal lobe of the caudal fin 
the two structures agree in 
number. Ventrally - prolonged 
haemal spines are the sole endo- 
skeletal supports of the inferior 
lobe of the caudal. The coraco- 
scapular cartilages of opposite 
sides remain distinct, and each 
supports a biserial fin. The 
pelvic girdle is represented by 
a pair of small cartilages sup- 
porting the basipterygia. The 
pelvic fins are uniserial, with 
post-axial skeletal supports for 
claspers in the males. Both 
the median and the paired fins 
are provided with marginal 
ceratotrichia. The skull is 
probably amphistylic. Five, 
possibly six or seven, branchial 
arches, bearing clusters of 
minute denticles, are present. 
Circumorbital plates are want- 
ing. All the endoskeletal struc- 
tures are partially calcified. The 
teeth are tricuspid, each with 
two long divergent lateral cusps 
Ua 
LE 
wn 
= 
SASA 
NG 
SSSA 
OSG 
\\ 
AWN 
Hy) 
NAT ODNOD 
a 
series of 
; B, basal cartilages of the paired fins; D, ceratotrichia ; DS, 
ar ; IC, interdorsal cartilages; MC, Meckel’s cartilage ; N, notochord ; 
A’, Ventral fin 
HM, hyomandibul 
arch ; P, supposed pelvic cartilage; the trian 
R, radialia of the paired fins; R’, rib; RB, radialia of the dorsal fin ; 
Fra. 250.—Restoration of Plewracanthus duchent. 
after Dean.) 
; HA, haemal arches ; 
head-spine 
gular cartilage behind it is the basipterygium ; PQ, palato-quadrate ; 
(From Parker and Haswell, 
G, shoulder-girdle. 
ay 
NA, neural 
and a minute median cusp; the broad bases of the teeth overlap 
and articulate with one another by means of facets. 
