Fishes of the Western North Atlantic 9 



toward the inner side of the curve.i* Skates also appear to employ the tail to some extent 

 in turning. But the chief method by which they turn, and the only method for the 

 members of the order in which the tail is either very slender or very short, is by inter- 

 rupting the undulations of the pectoral on the one side while those on the other side 

 are continued, or by varying the rates at which the waves of undulation pass along the 

 two pectorals ; the Ray swings to the right if the undulation is the more rapid on the 

 left-hand side thus driving the left-hand side ahead the more rapidly, or it swings to 

 the left if it is the right-hand undulation that is the more rapid. 



Breeding and Development. Fertilization is internal in all the batoids and is effected 

 in the same manner as among Sharks, i.e., by a pair of appendages, known as claspers, 

 that develop in the male from the inner edges of the pelvic fins. Their presence makes 

 the determination of the sex easy, for they are visible even in embryos shortly before 

 birth. The inner margins of the claspers are deeply grooved, with the edges more or 

 less overlapping for the transmission of sperm that is received at their inner ends via 

 an opening known as the apopyle. The primary support of the clasper consists of a 

 single basal cartilage connected with the basypterigial cartilage of the pelvic fin by 

 2-4 short intermediate pieces. During growth this basal element elongates while other 

 cartilages are formed secondarily from the surrounding connective tissue. Two of these 

 secondary cartilages, more or less elongate, lie alongside the primary basal element, 

 with which they fuse either wholly or partially to form a rigid axial rod. As maturity 

 approaches a series of additional terminal cartilages develop in varying numbers (2-5 

 or 6 in different groups), often in an exceedingly complex arrangement that exhibits 

 a wide range of forms, blade-like and knife-sharp in some cases though covered by a 

 thin layer of integument, or emerging from the skin as thorns in other cases. Some 

 of these terminal elements may be more or less erected at right angles to the general 

 axis of the clasper during copulation.^" 



The members of one family (Rajidae) are oviparous ; their eggs, enclosed in horny 

 capsules (p. 141), are familiar objects on the seashore. All other members of the order, 

 so far as is known (with one possible exception, p. 47, fn. 1 1), are ovoviviparous, the 

 embryos developing within the oviducts of the mother until they are ready for inde- 

 pendent existence. But there is no placental connection between young and parent. 



The embryos of the myliobatoid Rays as a group, and of some of the Electric 

 Rays, are nourished by a secretion from the vascular villi that clothe the inner wall of 

 that portion of the oviduct — the so-called uterus — where the embryo lies (pp. 91, 383, 

 397). But it is not known whether this is true of those rhinobatoids in which the uterine 

 wall bears only a series of longitudinal folds and no villi (p. 52) or of some Torpedo 

 Rays in which the structure of the uterus is similar. There is no reason to suppose that 



19. For recent discussion, with moving picture photographs showing successive positions assumed by a flexible-bodied 

 Shark [Scyliorhinus caniculus) in turning sharply, see J. Gray (Proc. roy. Soc. Lond., [B] 113, 1933: 118, pi. 4). 



20. For discussion and illustrations of the skeletal elements of the claspers in representative batoids, see especially Junger- 

 sen (Danish "Ingolf" Exped., 2 [2], 1898: pis. 3, 4; Anat. Anz., 14, 1898: 498-513). For detailed accounts of the 

 soft parts of the claspers, see Haber (Z. wiss Zool., yo, 1901: 627-634, pi. 28); for a more recent account (with 

 complex terminology for the various parts), see Leigh-Sharpe (J. Morph., 34, 1920: 260; 36, 1921: 213-218, 236- 

 243; 39' '9^: 560. 568-577; 42, 1926: 318-320). 



