THORNBACK. 
1105 
The length of the head from the tip of the snout 
to the occiput is about V5 of the entire length of the 
body. The length of the snout varies between 12 and 
13 % (exceptionally 14 %) of that of the body, or be- 
tween 17 and 1 9 1 / 2 % of the greatest breadth of the 
disk. The least interorbital width of the concave fore- 
head (exclusive of the dermal fold above the eyes) va- 
ries between 30 and 37 % (exceptionally 39 %) of the 
length of the snout. The longitudinal diameter of the 
oblong eyes is about 1 / 3 a of the length of the snout, 
and the longitudinal diameter of the spiracles, which 
lie just behind the eyes, is in young specimens only 
about % of that of the latter, in old Thornbacks almost 
equal to the same. The internasal width is in young 
specimens about 3 / 4 , in old 9 /io (exceptionally 95 %), of 
the distance between each nostril and the tip of the 
snout, or about 18 — 20 % of that between the cloaca 
and the last-mentioned point. The lateral flaps of the 
nasal valvule are fimbriated behind. The breadth of the 
mouth, which is transverse and but slightly curved, is 
equal in young specimens to the internasal width, some- 
what greater in old. The dentition differs considerably 
in the young and old and in the males and females. 
All young specimens have small, blunt, granular teeth, 
set in rows running obliquely on each side, and without 
any distinction between the sexes (fig. 315, A); but as 
the males approach maturity, the crowns of the teeth 
become more and more flattened, with the hind margin 
pointed and prolongated, a transition to the characteristic 
dentition of the old males. The teeth of the adult males 
are set in fairly regular rows, both longitudinal and 
transverse, and those forming the 14 — 22 rows in the 
middle of the mouth have a somewhat prolongated crown 
with subulate cusp (fig. 315, B), which is directed inwards 
towards the pharynx, thus lying like a scale over the 
tooth behind it. The rows nearest to the corners of 
the mouth have quite small, flat crowns, with the hind 
margin slightly prolongated into a flat, sharpened heel, 
which is more and more extended and approximated to 
the subulate form, the nearer the row to which it be- 
longs lies to the first-mentioned median rows. Only 
the teeth with subulate cusp have a very narrow rim at 
the base, and as these are most exposed to detrition, 
the tips of the foremost (oldest) teeth are always worn 
and blunt. In old females the teeth have broad, lo- 
zenge-shaped, and sloping crowns, set in pentagons 
(so-called mosaics, fig. 315, C ). This dentition is pe- 
culiar to the Thornback, and cannot be confounded 
with that of any other among the Scandinavian Rays. 
The whole upper surface of the body is densely 
strewn with fine, compressed spinulae, branching at the 
root into a star with two — four or even six arms. 
Similar small spines, more scattered, however, are set 
on the ventral side of the body in full-grown speci- 
mens, especially under the snout and tail, but are 
wanting on the pectoral and ventral fins, which are 
smooth. In young individuals, on the other hand, 
especially of the male sex, the whole ventral side seems 
smooth to the touch; but on careful examination very 
fine spines are detected even there, at least under the 
very tip of the snout and at the sides of the abdominal 
cavity. All the aculei belong to the type which, when 
D E 
Fig. 315. Jaw-teeth and aculei of the Thornback ( Raja clavata). 
A, teeth of a young male, natural size, and (below) a tooth magnified, 
lateral and superior aspects; B , tooth of an old male, magnified, 
lateral and superior aspects; C, tooth of an old female, magnified, 
aspects as before; D , dorsal aculeus of an old Thornback; E , caudal 
aculeus of a younger specimen. 
fully developed, has a smooth, expanded base ( aculei 
clavati). The ordinary consist in young specimens of 
1) a row running along the back, containing some 
twenty spines, with base as yet but conically expanded, 
2) an aculeus on the scapular cartilage, on each side 
of this row, 3) one or two before and two or three 
behind the eyes, and 4) a few quite small aculei on 
the rostral cartilage, set in two rows. In older speci- 
mens we find, besides those enumerated above, a row 
on each side of the median series at the root of the 
tail, and sometimes, especially in the females, two rows 
at the same point, containing aculei of the same size 
as or even larger than those of the median series. 
In a young specimen 14 cm. long the diameter of the eyes, according to Kkoyee, was 37 , /‘> / of the length of the suout. 
