Fishes of the JVestern North Atlantic 353 



Carolina, in Museum of Comparative Zoology; also a mounted female, about 1,250 mm 

 wide, from Woods Hole, in New England Museum of Natural History; and many 

 taken in local fish traps, examined at Woods Hole. 



Distinctive Characters. Large specimens of this species are recognizable at a glance 

 among Sting Rays of the western North Atlantic by their thorny tails, by the large 

 size and wide spacing of their mid-dorsal bucklers, and by the conspicuous tubercles 

 or bucklers on the outer parts of their discs. Smaller specimens, in which the large 

 tubercles have not yet developed on the tail, differ so widely in shape of disc from 

 Dasyatis sabina, D. guttata^ and Himantura schmardae that there is no danger of con- 

 fusing them with any of the latter. While D. centroura resembles D. say and D. ameri- 

 cana more closely in shape of disc, it is easily separable from D. say (if undamaged) 

 by the fact that its tail lacks any trace of a cutaneous fold above, and from Z). americana 

 by its much narrower ventral tailfold. Small D. centroura that have lost their tails might 

 be confused with either D. say or D. americana., so closely do the three species resemble 

 one another in general appearance prior to the full development of their dermal ar- 

 mature. However, the outer posterior corners of the disc are much more abruptly 

 rounded at all ages in centroura than in say (cf. Fig. 83 with 90), and the young cen- 

 troura remains smooth-skinned until after it has grown to a size at which americana has 

 commenced to show Its characteristic dorsal armature. The large size and broad spacing 

 of the mid-dorsal bucklers on the discs of older centroura afford a convenient field- 

 mark for identification, even for tailless specimens. 



Description. Proportional dimensions in per cent of extreme breadth of disc. Female, 

 610 mm broad, and male, 1,415 mm broad, from Buzzards Bay, Massachusetts (Harv. 

 Mus. Comp. Zool., Nos. 36508 and 3651 1 respectively). 



Disc: vertical length 82.3, 80.0. 



Snout length: in front of orbits 17.0, 15.7; in front of mouth 18.9, 16.2. 



Orbits: horizontal diameter 4.9, 3.0; distance between 10. o, 9.3. 



Spiracles: length 6.2, 5.4; distance between 16.2, 14.8. 



Mouth: breadth 8.4, 7.0. 



Exposed nostrils: distance between inner ends 10.6, 9.1. 



Gill openings: lengths, ist 2-2^ 2.8; 3rd 2.9, 3.0; 5th 2.2, 1.9; distance between 

 inner ends, ist 18.9, 18.4; 5th 12.8, 12.6. 



Pelvics: anterior margin 15.9, 14.5. 



Distance: from tip of snout to center of cloaca 73.0, 68.5; from center of cloaca 

 to origin of caudal spine 40.0, 41.0. 



Disc about 1.2 — 1.3 times as broad as long, rhomboid; tip of snout narrowly 

 rounded, projecting only slightly if at all; maximum anterior angle In front of spiracles 

 about 130—140°; anterior margins nearly straight or weakly concave but altering to 

 convex toward outer corners ; posterior margins nearly straight anteriorly but becoming 

 weakly convex posteriorly, making an angle of about 130—140° with anterior margins; 

 posterior corners abruptly rounded or even angular; Inner margins weakly convex. 



