REQNIDAE. 249 



derived from a common ancestral line nearer than that of most sharks to the 

 Platosomia, but the affinities are not very close. The common shape of members 

 of this family is that seen in fig. 1 of Plate 16. The entire form has been much 

 depressed and broadened; the pectorals, without being joined to the sides of the 

 head, have expanded forward and backward, and the dorsals, reduced in size, 

 are placed on the tail on which also supracaudal and subcaudal have become 

 modified. Branchial cartilages, shoulder girdle, and pelvis have lengthened 

 and strengthened and the dermal armature has undergone changes for special 

 protection demanded by habits of life. The gills are lateral, in front of 

 the pectoral fins, as in all Antacea. Of the dentition each tooth has a broad 

 backward extended base anteriorly on which rises an acute rather narrow some- 

 what compressed cusp with a sharp ridge on each side, continued to the edge of 

 the base and with an extension downward below the cone in front and another 

 backward behind it. There are generally no median teeth on the symphysis. 

 There are about twenty rows of teeth on each jaw of which three or four series 

 are in function. The scales vary greatly on the individual at different ages and 

 on different parts of the body. The pattern, best seen on the young, includes a 

 broad base, somewhat stellate in cases, from which rises a slender hooked cusp, 

 conical in the distal portion and ridged with four keels or more on the basal sec- 

 tion. Two keels extend down in front, with rarely a smaller median between, 

 and a stronger one passes down at each side; these again may be followed by one 

 or more. On some examples the keel at each side is stronger and reaches the 

 outer edge while a similar one goes directly backward, an arrangement suggestive 

 of the tooth pattern. At later stages the scales, especially along the middle of the 

 back, become more placoid or tubercular, but on the edges of the fins and on the 

 lower surfaces the crowns become leafshaped and imbricated. From the skull 

 and the branchial skeleton there is no doubt the family is to be classed with the 

 Antacea yet the affinities with the Rhinobatidae bring it closer than any other 

 of the sharks to the Batoids. The approximation is particularly noticeable in 

 the vertebral column, the neural spines, the calcification, the shoulder girdle, 

 the pectoral fins, the pelvis, in the ventral fins, and in the dorsals. In both Rhina 

 and Rhinobatus the dorsals are attached to modified neural spines, an arrange- 

 ment not found in the most primitive of the other recent sharks. Rhina and 

 Rhinobatus have like basalia in the dorsals and they have the same transverse 

 dilatations of the upper end of the neural spine at the origin of each dorsal fin; 

 in these respects both differ markedly from the Squalidae. 



Various fossil species from the Jurassic and later belong to the only genus 



