PAIRED FINS OF THE MACKEREL SHARK 57 



SESSIONAL PAPER No. 22a 



the basipterygial cartilages no doubt, but the disproportion is due no less to the large 

 development of the long cartilaginous rays. 



The cartilaginous fin-plate, as stated on a prior page, breaks up distally into rod-like 

 rays which by subsequent dichotomous division become extremely long and slender in 

 Lamna. At least six rays in the fin of the right side (Plate VI., fig. 2) have undergone 

 partial dichotomy distally, and in the left fin (Plate V., fig. 1) two rays show each at their 

 outer end a division into three, but the division extends merely for a short distance. 



The stout cylindrical piece at the upper anterior margin of the fin is the proptery- 

 gium. It has a conical nodular form, the apex being segmented into two or more distal 

 •elements, recalling the condition in Acanthias (Plate VII, fig. 4), and it articulates with 

 the pectoral arch by a concave facet, being held in place by strands of dense fibrous 

 tissue. The small rod-like cartilage on the outer margin of the propterygiura (Plate VI., 

 fig. 2a) is probably merely a migrating rudimentary ray, (in the .left fin this rod consists 

 of three segments, (Plate VI., fig. \a) the rays pushing their way in many species into the 

 basal series and, as in Torpedo and Trygon, separating the propterygium and the mesop- 

 terygium, or, as in Raia, separating the mesopterygium and the metapterygium (Plate VII., 

 fig. 9). Two such secondary basalia are present in MyliobcUes, leading some anatomists 

 to regard the mesopterygium as split into two. Closely articulating with the proptery- 

 gium is the somewhat regular quadrate mesopterygium (raesop.), a flattened plate of 

 cartilage in contrast to the stout cylindrical form of its more external neighbour (pro.) 

 This flat plate articulates by its two shorter opposite sides, on the one hand with the 

 propterygium, and on the other with the metapterygium (Plates V. and VI., figs. 1 and 2). 

 To its outer mai*gin six fin-rays may be attached, the first joints being irregular nodules 

 with which more is distally articulated in the right fin one larger cartilage, in shape like 

 an inverted L, and formed by the confluence of two rays at their base. Irregularity in the 

 division of the proximal portion of the first two mesopterygial rays is frequent, as in 

 Acanthias (Plate VII, fig. 4) and in Cestracion (Plate VII, fig. 7). 



In almost all the forms of pectoral fin referred to in this paper the metapterygium 

 (metap.) presents the character of a large elongated plate articulating with the mesop- 

 terygium (mesop.) by its anterior margin, and at its other extremity bearing a series of 

 irregular basal elements. If these nodules in Lamna. one of which has the form 

 rather of a flattened obquadrate plate, be simply parts segmented off from the metaptery- 

 gium, they would correspond to the two pieces shown in Wiedersheim's figure of the fin 

 of Heptanchus (Plate VII., fig. 5 x. y.). There is more reason, however, to regard the four 

 nodules {m.m.m.m.) at any rate as the detached proximal joints of the six adjacent rays 

 like the similar nodules at the anterior end of the mesopterygium (Plate VI, fig. 2 n. n.). 

 The intruding triangular fragment of cartilage (o.) may indeed be a fifth displaced 

 nodule of the series and the oblong bit (in.) on the left of the series may represent two 

 such coalesced terminal nodules. There is every reason to regard the three elements 

 (metap. o. and q.) as metapterygial, and the metapterygium thus bears a total of no less than 

 twenty-two fin-rays, the mesopterygium carries only six, and the propterygiura one or, at 

 the most, two rays. The distal termination of the 19th (or it may be the 20th) ray 

 (Plate VI., fig. 2) shows a peculiar bifurcation, so that it ends not in one or two digiti- 

 form points but in no less than four, three of them distinctly dactyliform. The nodule 

 marked Z may be the displaced terminal segment of 19, as 18 may be the similar 

 displaced piece froi the 18th ray. The remaining eleven rays are all markedly digiti- 

 form excepting the 25th, 26th, 27th and 30th, which have no terminal acuminate 

 nodule such as the others possess. Similar distal segments are seen in the fin-rays of 

 Scyllium, Heptanchus and Chimoira (PI. VII., figs. 3, 5 and 6), though the reduction in 

 the cartilaginous skeleton of the fin of Scyllium is such that the hexagonal, or rather, 

 somewhat geometrical polygonal nodules, around the margin of the series of rays, may 

 represent not the digitiform elements of Lamna or Chimcera, but the last two segments. 

 The segmentation of the rays in Lamna is not wholly regular, though three rod-like por- 

 tions are segmented off in most, and there is, on the whole, a regular uniformity in this 

 feature. Some rays exhibit an additional terminal nodule, and a number exhibit partial 

 longitudinal and false transverse segmentation. The small cartilaginous rod lying just 

 outside the propterygium in the right fin(Pl. VI., fig. 2, a.) and the pair of two-jointed 

 rods occupying a parallel position in the left fin (PI. V., fig. 1, a.) are, as already indicated 



