Fishes of the Western North Atlantic 171 



Caudal usually a little more than V2 the total length," its upper lobe narrow, scythe- 

 shaped, with a small rounded subterminal prominence but no definite subterminal notch, 

 its lower anterior corner produced as a small but definite subtriangular lobe, the lower 

 anterior margin about V^ to H as long as the upper margin, its axis raised only slightly. 

 Anal similar to 2nd dorsal in size and shape, its origin posterior to rear end of base of 2nd 

 dorsal by a distance as long as its own base, or a little longer. Pelvics about as large in area 

 as 1st dorsal, with weakly convex anterior margins, rounded apices, moderately concave 

 distal margins and subacute inner corners. Claspers of adult males about 4 times as long 

 as pelvic fins and very slender. Pectoral nearly or quite twice as long as height of ist dorsal 

 along anterior margin, falcate, with very broad base, the anterior margin rather strongly 

 convex in small specimens but tending to become less so in large,'' the apex broadly 

 rounded, the distal margin deeply and evenly concave, the inner corner subacute, the 

 inner margin only a little more than V2 as long as breadth of base. 



Color. Back and upper sides varying between brown, blue slate, slate gray, blue gray 

 and dark lead, even nearly black, often with metallic luster; shading along sides to white 

 below, except that lower surface of snout in front of nostrils, as well as lower surfaces 

 of pectorals, may be of same hue as upper sides; white of lower surface reaching farthest 

 upward on sides from axil of pectoral to opposite rear part of ist dorsal and again rear- 

 ward from pelvics j the sides near pectorals, the lower surface from pelvics to caudal, and 

 sometimes the belly may be more or less mottled with gray; iris black or green. 



Size. Maximum length 20 feet or more, with lengths of 13 to 16 feet common. 

 Judging from the sizes of females with embryos and of males with large claspers, sexual 

 maturity probably is not attained at a length less than 14 feet. A female of 4,410 mm. ( 14 

 feet 6 inches) was found to contain an embryo of 1,550 mm. (5 feet i inch)." On the other 

 hand, a free-living specimen as small as 46 inches has been reported, while many of 4 to 5 

 feet have been taken at Woods Hole. One of 149 cm. (about 4 feet, 10Y2 inches) taken 

 there is described as still showing the umbilical scar,^° but no trace of it is to be seen on 

 another slightly smaller specimen (4 feet 4 inches) which we have examined from the 

 same locality. The few recorded weights range from about 300 to 320 lbs. at lO feet, 375 to 

 400 lbs. at about 13 feet, and about 500 lbs. at 14 feet 5 inches, up to a maximum of per- 

 haps 1,000 lbs. 



Develof mental Stages. No information is available about the embryo, except that 

 the caudal is about as long, relatively, as in the adult. But the decrease in relative size of 

 the eye with growth after birth suggests that it is even larger in the embryo, as is so com- 

 monly the case. Apparently the number of young in any one litter is much smaller than 

 in many other ovoviviparous species, for females have been reported as containing two 



17. Ratio of caudal length to trunk length (snout to origin of caudal) ranges from i.i:i to 1.3:1 in specimens 

 examined from Massachusetts, San Francisco and the west coast of South America, but only 1.04:1 in a 

 Mediterranean example measured by Tortonese (Atti Soc. ital. Sci. nat., yy, 193S: 293). 



18. Owing to the obliquity of their basal insertions, the pectorals often appear as straight-edged, or nearly so, in 

 photographs of large Threshers suspended by the caudal peduncle, as we have observed. 



19. Poey, An. Soc. esp. Hist. Nat., 5, 1876: 383. 20. Springer, Copeia, 1943: 55- 



