6 Memoir Sears Foundation for Marine Research 



positions of the gill openings, and in a number of skeletal characters, of which the follow- 

 ing may be selected as the most obvious : 



The upper jaw cartilage is attached only loosely to the cranium, at most, by a 

 ligament of considerable length. 



The cranium is firmly connected with the vertebral column by a definite articul- 

 ation between its posterior face and the anterior vertebra by means of two condyles which 

 are lacking in Sharks. 



The first few vertebrae, difi^ering in number in different families, are united together 

 as a continuous rigid tube.* 



The ceratohyal cartilage is attached only to the lower end of the hyomandibular 

 and thus plays no direct part in the support of the lower jaw, which is suspended from 

 the hyomandibular alone. 



The shoulder girdle is directly and firmly attached to the vertebral column, either 

 above the latter by a special scapular element (or elements) or to its sides. 



The propterygial cartilage of the pectoral bears at least as many radials as the 

 metapterygial cartilage and many more than the mesopterygial, which is much smaller 

 than either the propterygial or the metapterygial. 



The union of the two halves of the upper jaw at the symphysis is much more 

 intimate in many batoids than in Sharks, but not in all.' On the other hand, the con- 

 nections between the anterior parts of the upper jaw cartilages and the cranium, being 

 by ligament only, are less intimate among the batoids than they are in many Sharks, 

 though the two groups intergrade in this respect.^ 



It has long been known that the attachment of the pectorals to the sides of the 

 head in batoids is a secondary development, for the early embryos of even the most 

 highly specialized of them are slender-bodied like those of Sharks, with the rudiments 

 of the pectoral fins at first wholly posterior to the gill openings. As development pro- 

 ceeds, the pectorals expand rearward as well as forward past the gill openings in a 

 form that has been variously described as blade-like or horn-like. The anterior horns 

 of the pectorals then fuse with the sides of the head above the five posterior gill openings 

 that are destined to persist as such but below the persistent portion of the first embryonic 

 gill openings that are destined to form the spiracle of the adult. The sequence of events 

 of this transformation of the pectorals has been observed among Skates (Rajidae), 

 among Torpedoes (Torpedinidae),* and among Sting Rays. 



6. For excellent illustrations of the anterior part of the vertebral column showing this fusion, see Garman (Mem. Harv. 

 Mus. comp. ZooL, j6, 1913: pi- 55, figs. 3-10). 



7. This is true of the Rajidae but not of the genus Gymnura (Dasyatidae), in which the two halves of each jaw cartilage 

 are clearly separated. 



8. For further information on this point, see Bigelow and Schroeder (Mem. Sears Found. Mar. Res., I (i), 1948: 65). 



9. The sequence of stages leading to the fusion of the anterior parts of the pectorals with the sides of the head seems 

 to have been observed first by Leuckart (Z. wiss. Zool., 2, 1850: 261, pi. 16, figs, i, 2) in the Mediterranean Torpedo 

 marmorata more than a century ago. For other illustrations of embryo torpedinids with the anterior parts of the 

 pectorals still wholly or partially free, see de Sanctis (Atti Accad. Sci. fis. mat. Napoli, 5, 1873 : pi. i, fig. 9), Goodrich 

 (in Lankester, Treat. Zool., 9, 1909: 754, fig. 117), Prashad (Rec. Indian Mus., 19, 1920: pi. 7, figs. 7, 8), and espec- 

 ially Waite (Rec. Canterbury [N. Z.] Mus., i [2], 1909: pi. 18, fig. 3); see also p. loi. Fig. 23. The process was de- 

 scribed and pictured in some detail by Wyman (Mem. Amer. Acad. Arts Sci., [N. S.] 9, 1867 : 35, i pi.) in his classic 



