PICKED DOG-FISH. 



1159 



(accordinir to Kroyek 16 — 17 ?a) of the Iciijrth of the 

 head to the first gill-openiiig. Like the 'ro|)e, the 

 Pieked Dog-fish has an almost terete body, but the 

 anterior part of the trunk is of the so-called hog-backed 

 form, with a rounded triangular section. iSeliind tlie 

 body is slighth' compressed, the breadtii being al)out 

 eipial to the depth, and the section almost quadran- 

 gular, though rounded (convex) abo\e and laterally 

 siiarpened below bv a dermal carina, running on each 

 side b(;hind the perpendicular from the second dorsal 

 fin and some way out above the beginning of the in- 

 ferior caudal lolie. The sides of the tail are, howevei-, 

 without true median carina, nor are there any caudal 

 grooves, though a trace tliereof mav sometimes l)e 

 found above, at tlie dorsal margin, in front of the 

 caudal fin. Tlie head, in particular the translucent 

 snout, is depressed, behind somewhat broader than the 

 trunk, with l)readth about equal to the deiith of the 

 latter, and with parabolically pointed snout. 



Tile length of the head to the first branchial aper- 

 ture varies between 17 (in the nearly viable foetus 18) 

 and 15 % of the length of the body. The length of the 

 snout to the eyes measures about "3 (40 — 44 S) of the 

 length of the head. Both the eyes and the orbits are 

 oblong, the latter most so, the vertical diameter of the 

 eyes being always more than '/o (53 — 70 %), that of 

 the orbits sometimes V3 (-11 — 33 %"), of the longitudi- 

 nal diameter. This measures in the case of the latter, 

 which are besides sharply pointed at each end, about 

 30' — 20 % of the length of the head. The pupil is 

 either ronnd, but acute-angled below, or transversely 

 oblong. Behind the eyes, but higher (further in) than 

 these, lie the ratlier large spiracles, their diameter being 

 only slightly less than or even equal to the vertical 

 diameter of the orbits, from which they are separated 

 by a distance of the same magnitude. Their anterior 

 margin is folded double, the fold being sometimes so 

 deep as to give them the appearance of being really 

 double. The interorbital width is generally about equal 

 to the length of the snout, somewhat greater or less, 

 usually the latter. The prone nostrils are set about 

 half-way along the snout, but the distance from the 

 mouth to the transverse line drawn through their pos- 

 terior margins is greater in the young than that from 

 the tip of the snout to the transverse line drawn through 



their •nntcrior margins. In the old the said distances 

 ;ire aliout ei|uai, the lip nf tiie snout in front of the 

 nostrils being even relatively prolongaterl with age. 

 The nostrils are, as usual, obliquely transversal, with 

 a small ])ointcd valvule at the middle of tli(> anterior 

 margin. Their width is about '3 of the distance be- 

 tween them. The mouth is less curved than in most 

 other Sharks, with deep folds around the corners. The 

 distance from the tip of the snout to the anterior mar- 

 gin of the mouth is about ' „, to the corners thereof 

 about 60—70 %, of the length of the head. The teeth 

 are densely set, without gaps or unpaired median tooth. 

 They are merely attached to the skin, as in all tlie 

 Plagiostoms, not to the cartilage, and are c(jnse([uently 

 mobile. The first two rows of teeth stand ujiright, 

 apart from the rest, the inner row leaning backwards; 

 the remaining 3 or 4 rows have the cusps turned inwards 

 (backwards), covered l)y the transverse fold (\elum) in the 



Fig. ,^)38. One of the right teeth of the upper jaw (n) and the lower 

 jan- (/;) in tlie Picked Dog-fish (Squalus acatithias), anterior aspect. X 5. 



front part of each jaw. The last-mentioned ro^^•s are, 

 however, erected in the old, where the anterior rows 

 have disappeared. Each tooth (fig. 338) is small — in 

 an adult fish only about 2 mm. high — flat and sharp- 

 edged, with a deep jag in the outer (posterior) margin, 

 under the keen, laterally directed cusp. Thej' have the 

 same form in both jaws, but are somewhat larger in 

 the lower than in the upper. In front they have a 

 basal cusp running into the gum, and reminding us of 

 the median ridge and the shaft with which the der- 

 mal scales are furnished. In a young female Suxdevall 

 counted 26 teeth in each transverse row above and 

 22 below. The tongue is not free, and consists mere- 

 ly of a skin investing the rather large, liyoid carti- 

 lage, whicii occu])ies almost the whole mandibular arch. 

 The branchial apertures are comparatively small, the 

 middle ones smallest, the hindmost opening somewhat 

 larger than the first, the height of M'hich is about '3 

 to ' „ greater than the vertical diameter of the orbits. 



" According to Kroyeu down to 28 ; 

 ' In a nearly viable fcetus rather mo 



