178 ORIGINAL ATtflCLES. 



by whicli to wield its vertebral column as an instrument of pro- 

 gression, is, apart from the undulatory movements of its short tail, 

 which probably acts principally as a rudder, entirely dependent on 

 its dorsal and anal fins. But a mere lateral movement of these, 

 caused by the contraction of a whole muscular mass first on one side 

 and then on the other, would not produce progression ; and indeed 

 (as it appears to me) the integument is too unpliable to admit of 

 such contraction. The movement is doubtless a feathering one, 

 a combination of flexion and rotation ; and, that Avhich determines 

 the movements so as to produce progression is the greater strength 

 of the muscular masses anteriorly. It may be conceived thus : let the 

 most posterior muscles of the dorsal fin on one side be contracted, 

 and at the same time the anterior muscles of the opposite side : the 

 former will fix the hinder angle of the fin to their own side of the 

 joint ; while the latter, being much stronger, will not only drag the 

 anterior angle of the fin to the other side, but will also flex it, so that 

 the fin will strike the Avater backwards. Let us now suppose that 

 the muscular contraction passes forwards in a wave on one side, and 

 at the same time backwards on the other, and so round and round : 

 the fin will administer a series of strokes backwards, alternately on 

 one side and the other. If the anal fin be moved in the same 

 manner so as always to be vertical to the dorsal, the action will be a 

 very regular one. Wlien we understand that the fins are thus moved 

 by a continuous wave of contraction travelling along their muscles, 

 we are also enabled to see the advantage of the individual muscles 

 being expanded and sheathed each one within that in front : for, by 

 this an'angement, the fibres contracted at any one moment are 

 scattered over as great a space as possible ; the muscular mass 

 will therefore midergo but little change in form as the wave passes 

 along it, and will so accommodate itself to the imyielding limits of 

 the space in which it is contained. 



There is an additional small muscle in connexion with the dorsal 

 fin, so small that its action nuist be very slight. (Pt. V. fig. 1, e.) 

 It is so closely in contact, in the middle line, with its fellow of the 

 opposite side as to be in appearance one with it, and stretches from 

 the tip of the supraoccipital to the first dorsal interspinous bone. 

 Mr, Tiu-ner pointed it out to me : he had described it in his notes. 

 It consists of a small muscular belly, prolonged back into a long 

 tendon, like the plantaris in man, and exists with the same attach- 

 ments in other fishes, only in them it is muscular throughout. 



There are only two small vestiges of the abdominal muscles, so 

 small that they have been hitherto ujidetected. One of them, 

 (Pt. VI. fig. 2, e) arises on the lower half of the shoulder-girdle from 

 the " idna : " its fibres were, in the specimen now described, less than 

 two inches long ; and the majority terminated abruptly on the mem- 

 brane over the abdominal cavity ; only a very few of the superior 

 ones being continued into meagre tendons which passed up to the 

 " clavicle." The other abdominal muscle (Pt. VI. fig. 2, d), 



