140 Me^noir Sears Foundation for Marine Research 



fold along either side. Pectorals of ordinary form, without spatula-like lateral process; 

 outer posterior corners more or less broadly rounded, their posterior outlines definitely 

 recurved before their junction with the sides of the trunk, the fin thus having a distinct 

 inner posterior margin; pectorals overlapping pelvics somewhat in most cases. Two 

 dorsal fins." Caudal reduced to a narrow finfold, sometimes lacking in adults. 

 Outer margins of pelvics more or less concave but not cut deeply enough to separate 

 anterior from posterior section of fin. Eyes with upper part of pupil covered by a dark 

 fleshy veil or velum, with crenulate margin that can be expanded or contracted (con- 

 trols amount of light entering pupil). Nostrils strongly oblique. Teeth either closely 

 crowded in quincunx arrangement or more loosely spaced in transverse series; rounded 

 in most females, cuspidate in mature males, and both sexes of some species^* with 

 sharp cusp. Upper surface of disc with prickles or larger thorns, or both. Anterior 

 margin of cranium with a distinct rostral projection (rostral cartilage) reaching nearly 

 or quite to tip of snout, extending at least as far as extremities of pectoral rays or far- 

 ther (see discussion p. 135). Anterior lobe of pelvic supported by three radials artic- 

 ulated along posterior bar of pelvic arch, followed without intervening gap by first 

 2—4 radials borne along anterior part of basipterygial cartilage. 



Young Skates at hatching are already so much like the adult in form that they 

 are recognizable as to species, at least In most cases (for an exception, see p. 148; Key 

 alternative 7 b). However, they may differ more or less from their parents In form of 

 disc. In details of spination, and in possessing a somewhat longer tail. There is only 

 one juvenile character that calls for special mention here, and that is the persistence for 

 some little time after hatching of a greater or lesser part of the long caudal filament 

 that is characteristic of embryo Skates in general during late embryonic stages. Dif- 

 ferent species, and possibly different individuals even within a given species, appear 

 to differ in this respect. Consequently, the length of the tail to the origin of one or the 

 other of the two dorsal fins Is a more dependable character for diagnostic purposes than 

 is the tail's total length. 



The electric organs, of species for which they are known, are slender spindle- 

 shaped masses along the sides of the tail, extending nearly the entire length of the 

 latter in some species but shorter In others. They also vary in thickness from bulky, 

 occupying practically the entire space between the skin and the vertebral column, to 

 much narrower and separated from the vertebral column by a thick muscular layer. 

 In some species the electric elements consist of columns of discs at right angles to the 

 longitudinal axis of the tail, whereas in others they are small club- or cup-shaped bodies 

 with a longer or shorter tail-like prolongation. They have a rich blood supply and are 

 Innervated with Innumerable fibrils which are derived from spinal rather than cranial 



27. Abnormalities in this respect have been described; see under R.garmani, p. 204, footnote 14. 



28. The alteration that takes place in the shape of the teeth in males of many species at sexual maturity results from the 

 replacement of the older rows by younger ones, not from any alteration in the shape of the teeth present earlier. 

 Examination of specimens approaching maturity also shows that this replacement takes place a little earlier in the 

 upper jaw than in the lower. 



