TIIOHNliACK. 



nor) 



The lenijtli of tlic licnd tVoiu the tip of tlio snout 

 to till' (H'ciimt is alioiit ' ., of tiie ciitii'c Iciigtii of tiu' 

 body. Tlie Juiigtii of the snout \ari('s between \'2 and 

 13 ?^ (exceptionaJh' 14 %) of tliat of tiie l)ody, or l)e- 

 tween 17 and li)' „ % of the greatest breadth of the 

 disk. Tiie least interorbital width of the concave fore- 

 head (e\ciusi\e of tlie dermal fold aboxc the (ncs) va- 

 ries between 30 and M7 % (exceptionally 39 %) of the 

 length of the snout. The longitudinal diameter of the 

 oblong eyes is about '/V' "J^ ^^^^ length of the snout, 

 and the longitudinal diameter of the s])iraeles, which 

 lie just behind the eyes, is in young specimens only 

 about ^:.. of tliat of the latter, in old Thornbacks almost 

 equal to the same. The internasal width is in young 

 specimens about ^ .,, in old " ,„ (exceptionally 95 %), of 

 the distance between each nostril and the tip of the 

 snout, or about 18 — '20 ?6 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 

 ill the young and old and in the males and females. 

 .\11 young specimens have small, blunt, granular teeth, 

 set in rows running obliijuely on each side, and \vithout 

 any distinction between the sexes (fig. 'S\ri,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, /?), wdiich 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, tig. 315, C). This dentition is pe- 

 culiar to the Thornback, and cannot be confounded 

 with that of an\' other among the Scandinavian Kaj's. 

 The whole uppei- surface of the body is densely 

 strewn with fnie, coni])ressed spinuhe, branching at the 

 root into a star with two — four or even six arras. 

 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 jxictoral and ventral fins, which are 

 smooth. In Aouiig 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 alxlominal 

 cavity. All the aculei belong to the type which, wlien 



; Y a 



D 



E 



Fig. 315. .Jaw-teetli and aculei of tliu 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; U, 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 

 clavaft). The ordinary consist in young specimens of 



1) a row running along the back, containing some 

 twenty spines, with base as j^et but conically e.xpanded, 



2) an aculeus on the scapular cartilage, on each side 

 of this row, 3) one or two before and two or thre*! 

 behind the eyes, and 4) a few quite small acvilei 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 ro\vs 

 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 Kroyee, was 37'/.^ % of the length of the snout. 



