2 I 8 Memoir Sears Foundation for Marine Research 



also many specimens up to about five feet long examined by us in the field between 

 Chesapeake Bay and eastern Maine. 



Distinctive Characters. The midbelt of the disc of R. laevis, from the level of the 

 axils of the pectorals to the vicinity of the spiracles, lacks large thorns, which sets 

 it apart from all other Skates of the genus Raja of the western North Atlantic, except 

 for R. mollis and R. spinicauda. And its thorny tail differentiates it from mollis. It is 

 easily separable from spinicauda (at least from the time it is one-third grown) in that 

 its tail is armed with three rows of thorns (only a single median row in R. spinicauda), 

 and that the mucous pores on its lower surface are marked by conspicuous black dots 

 and dashes (not so in R. spinicauda or in any other Raja of the western North Atlantic). 

 Newly hatched specimens of R. laevis, on which the mucous pores are not yet pig- 

 mented (p. 222) and on which only one row of thorns has developed along the tail, 

 may prove difficult to separate from R. spinicauda at a corresponding stage. The young 

 of the latter have not been seen. R. laevis is so closely allied to the well known Gray 

 Skate (R. batis) of the northeastern Atlantic, which also has the mucous pores of the 

 lower surface marked by black dots and dashes, that future study may show that the 

 two represent extremes of one varietal series. However, present indications are that 

 the teeth average fewer in R. laevis (30-40) than in R. batis (44-55), those of females 

 being relatively lower and more rounded; the tail may average a little longer in R. 

 laevis; the lateral rows of thorns along the tail are more regular (often largely lacking 

 in R. batis). On the other hand, the lower surface of the disc of some specimens is 

 more generally prickly on R. batis.*^ 



The only Skates known in the western Atlantic, other than R. laevis, which have 

 pigmented mucous pores on the lower surface are R. castelnaui Ribeiro 1907" and 

 some specimens of i?. agassizii Miiller and Henle 1841, both from Brazil;*^ R.platana 

 Giinther from Rio de Janeiro to the mouth of the Rio de La Plata; and R. flavirostris 

 Philippi 1892 from the Patagonian-Falklands region and Straits of Magellan to Chile. 

 The anterior contour of the disc is so much more obtuse in R. castelnaui, and the dorsal 

 fins are so much farther removed from the tip of the tail in R. agassizii, that R. laevis 

 is not likely to be confused with either of these species. R.platana and R. flavirostris 

 resemble R. laevis closely in general form. But R. flavirostris is characterized by the 

 presence of 2-3 strong thorns around each orbit and one large nuchal thorn, which 

 are lacking on R. laevis. And in R.platana the median thorns on the tail (18 on spe- 

 cimen studied) are not only fewer and much larger than in R. laevis but are successively 

 larger rearward; there are no side rows on the tail; the space between the eyes and 

 also the mid-dorsal belt in the nuchal region (smooth in R. laevis) are close-set with 

 small, low, rounded tubercles; and the caudal extension, posterior to the second dorsal 

 fin, is considerably longer relatively than in laevis. 



4j. We have for comparison two large R. batis from Sweden. 



44. Arch. Mus. nac. Rio de J., 14, 1907: 177, pi. 15. 



45. Some of the specimens of R. agassizii that we have examined have the mucous pores pigmented on the rostrum, 

 around the mouth, and in the area between the gill openings, but others are immaculate below except for a sooty- 

 blotch under the tip of the snout. 



