78 



DESIGN IN NATURE 



PLATE XLVIII (continued) 



propellers. Tliese are constructed on the principles of true wings, being triangular in sliape and graduated ; the root and anterior 

 margins of each flipper being thick and semi-rigid, while the tip and posterior margins are thin, flexible, and elastic. This peculiar 

 structure, when the flippers were made to vibrate, would produce a rapid forward motion akin to flying, and the speed of the Plesiosaurus 

 must have been as phenomenal among animals as that of the dragon-fly among insects. The flippers represent modified arms and legs, 

 the fingers and toes consisting of five tapering rows of small, slightly elongated, square-shaped bones. As the flippers are loosely jomted 

 at the shoulder and pelvis by universal joints, and all the bones composing them are free to move, the range and variety of motion must 

 have been very great. The movements, there is reason to believe, consisted of vertical vibration with a certain amount of rotation 

 along the anterior margins of the flippers, accompanied by triangular twisting of the flippers, as happens in wings. The flippers some- 

 what resemble those of the sea bear (Otaria jubata). Compare -H-ith Plate Hi., Fig. 1. 



Fig. 3.— Skeleton of the great old-world fish-reptile (Ichthyosaurus tenuirostris), (after Cuvier). Shows the same features as in the 

 Plesiosaurus just described, with the following differences. The four flippers are shorter, broader, and less highly developed as 

 propelling organs. To remedy this, the animal was provided with a powerful swimming tail, believed at one time to have had no 

 caudal fin. From a specimen, however, discovered in the Lias of Wurtemburg, where the contour of the soft or fleshy parts could be 

 made out, a caudal fin was present, the terminal portion of the spinal column occupying its lower lobe, while in the sharks it 

 occupies the upper lobe. Professor R. Owen, with remarkable sagacity, ])redicted the presence of a caudal fin in the Ichthyosaurus. 

 The tail of the Ichthyosaurus, there is reason to believe, was made to vibrate vertically in swimming, as in the dolphin, whale, 

 dugong, manatee, &c., and not laterally as in the fish. This is almost certain from the number, position, and configuration of the ribs, 

 and the nature of the breathing, which was aerial. 



Fig. 4.— Recent and extinct cuttle-fishes (after Zittel and Mantell). Show longitudinal, radiating cleavage and transverse 

 markings. 



A. Modern cuttle-fish from Pacific ocean (Enoploteuthis leptura). a, arms ; 6, tentacles; c, mouth ; d, eyes ; e, funnel; /, mantle. 



B. Internal shell of ditto. 



C. Fossil cuttle-fish partly restored (Belemnoteuthis cmtiqua). a, arms ; b, eyes ; c, mantle ; d, ink-bag ; e, phragmacone. 



D. Ink-bag of ditto. 



E. Fossil cuttle-fish (Plesioteuthis prisca). 



F. Shell of ditto. 



PLATE XLIX 



Plate xlix. illustrates longitudinal, radiating, and transverse cleavage and expansion in relation to the organs 

 of locomotion. 



Fig. 1. — Skeleton and outline of the deer (Gervus duph us), (after Pander and D' Alton). Shows longitudinal and transverse cleavage 

 of limbs and the small feet adapted for land transit. In this fleetest of animals the angles made by the bones of the limbs with each 

 other and with the shoulder (scapula) and hip (pelvis) are comparatively acute, and afford facilities to the muscles for suddenly 

 shortening and elongating the limbs in rapid progression, a, Angle made by the femur with the innominate bone (pelvis) ; b, angle 

 made by the tibia and fibula with the femur ; c, angle made by the cannon bone with the tibia and fibula ; d, angle made by the 

 phalanges with the cannon bone ; e, angle formed by the humerus with the scapula ; /, angle made by the radius and ulna with the 

 humerus ; g, angle made by the camion bone with the radius and ulna ; h, angle made by the phalanges with the cannon bone. 



Fig. 2. — Thresher or fox shark (Carcharias vulpes). Shows large pectoral and caudal fins, indicating great speed in swimming. 

 The pectoral fins are true wings as regards construction ; that is, they are triangular in shape, thick and semi-rigid at the root and along 

 the anterior margins, and thin and elastic at the tips and along the posterior margins. When made to vibrate in a vertical direction 

 they act as powerful propellers. The huge tail, which is the principal swimming organ, is similarly constructed. Photographed for the 

 Author by B. Millar. 



Fig. 3. — Feet of swan and grebe. Show longitudinal cleavage and radiation in the bones of the feet. 



A. Legs and feet of the swan (Gygaus), with the latter closed and spread out as when making the non-eft'ective (a.) and 

 effective stroke (6) in swimming. The closing and opening of the feet enable the bird alternately to evade and seize the water very 

 effectually. Drawn from nature for the Author by C. Berjeau. 



B. Foot of the grelje (Podiceps), (after Dallas). In this foot each toe is provided with a swimming membrane, the membrane being 

 closed when the foot is flexed and making the non-effective stroke, and expanded when the foot is extended and making the 

 effective stroke. 



Fig. 4. — Hind extremities or posterior flipper of the elephant seal (Macrorhinus leoninus) in the closed and expanded condition. 

 (Challenger 'Rtpovts, vol. xxvi.) Show cleavage and radiation of bones of feet. The flippers in swimming are alternately opened and 

 closed, as also happens in the tail of the fish when making the non-effective and eflective strokes. Each flipper consists of five toes 

 supporting a flexible swimming membrane. Compare with A and B of Fig. 3 of this Plate. 



Fig. 5. — Heterocercal or unsymmetrical tail of the sturgeon (Acipenser sttirio), (after Giinther). Shows division and radiation 

 resulting in a beautifully-graduated swimming organ. The terminal, tapering portion of the spinal column occupies the up])er lobe of 

 the tail, and the tail is thick at the root and thin at the free margin (/, g, h). It is also thicker and stronger above (a, 6, c), and thinner 

 and more flexible below (d, e). It bears a considerable resemblance to a wing. In reality, caudal and other fins, flippers, and wings are 

 constructed on a common type, and discharge essentially similar functions. 



Fig. 6. — Skeleton and outline of the ostrich (Struthio camelus), (after Dallas). Show longitudinal and transverse cleavage in legs, 

 wings, vertebral column, and riljs. Remarkable for its large, powerful legs, small feet, and rudimentary wings — conditions necessary 

 for rapid land transit. What is said of the limbs of the deer (Fig. 1 of this Plate) applies to the ostrich, a, Angle made by the femur 

 with the pelvis ; b, angle made by the tibia and fibula with femur ; c, angle made by the tarso-metatarsal bone with the tibia and 

 fibula ; d, angle made by the bones of the feet with the tarso-metatarsal bone ; e, /, bones of aborted wing making nearly right ancles 

 with each other. ° 



