4 Memoir Sears Foundation for Marine Research 



Keys. The Keys are offered solely for purposes of identification. Therefore, we have 

 selected for their construction such characters as are not only alternative but which are 

 easily seen or measured. 



Arrangement. The larger groups, down to genera, are arranged in the sequence that 

 seems to us to represent most nearly their probable relationships. The species within 

 each genus are presented in alphabetical sequence as in our previous publication. Sharks.^ 



References. All citations were consulted in the original, except as noted, through 

 the co-operation of the several libraries listed previously.^ 



Sources of Material. The collections in the Harvard Museum of Comparative 

 Zoology and the United States National Museum have been the chief sources of our 

 Study Material. We are much indebted also to the museums and persons listed (page i). 



Proportional Dimensions and Illustrations. The measurements from which the pro- 

 portional dimensions of the several species have been calculated were taken on a hor- 

 izontal line between perpendiculars at given points ; for example, the length of the snout 

 in front of the orbits is on line BC in Fig. I, not AC ; the length of the disc is measured 

 on line BC, not AC ; the anterior angle to the level of the spiracles is the angle DAE. 



The great majority of the drawings in this book were prepared by the well known 

 zoological artist, E. N. Fischer, who executed most of the illustrations for Part I of 

 this Memoir series and for Carman's Plagiostoma (19 13). Ceratobatis robertsii., the type 

 specimen of which is in the British Museum (Natural History), was drawn by Hubert 

 Williams, and original drawings of Diplabatis pictus were loaned to us by courtesy of 

 G. Palmer. 



Subclass ELASMOBRANCHII 



Order BATOIDEI 



Sawfishes, Guitarfishes, Skates and Rays^ 



Characters. In the Batoidei the gill openings are wholly on the ventral surface. 

 The anterior edges of the pectorals are united with the sides of the head forward past 

 all five pairs of gill openings, about to the level of the mouth in some (Pristidae), to the 

 level of the nostrils in others, to the tip of the snout in still others. The upper edges 

 of the orbits are not free from the eyeballs, as they are in Sharks; that is, they do not 

 have free upper eyelids. In these respects they differ from all modern Sharks. None 

 have nictitating membranes, anal fin, or precaudal pits or furrows. 



The great majority of the batoids are easily recognizable by their shapes. Their 

 trunks are strongly flattened dorsoventrally, with the pectorals widely expanded so that 

 they are disc-like in shape. The tail sector is more or less distinct from the body sector, 

 the eyes and spiracles are on the dorsal surface, and the mouth, as well as the entire 

 lengths of the gill openings, is on the ventral surface. 



2. Bigelow and Schroeder, Mem. Sears Found. Mar. Res., I (i) Chap. 3, 194S. 



3. Mem. Sears Found. Mar. Res., I (i), 1948: 60. 



4. For characters of the order Selachii, see Mem. Sears Found. Mar. Res., I (i), 1948: 62-64. 



