458 FISHES CHAP. 
are usually confluent with the sides of the head. Tail slender, 
sharply marked off from the trunk, to which it usually appears 
as a mere appendage. Dorsal fins, when present, on the tail. 
Anal fin absent. Branchial clefts ventral in position. Spiracles 
large, usually crescentic. Vertebrae tectospondylic. 
For the most part the Batoidei are sluggish ground- Fishes, 
slowly moving over the sea-bottom by the gentle undulatory 
vibrations of the margins of their huge pectoral fins, the tail 
being of little use in locomotion. They feed principally on 
Crustacea, Molluses, and the smaller Teleosts. As with other 
Fishes of similar habits, the coloration of the dorsal surface 
harmonises with that of the sea-bottom, while the ventral surface 
is either deficient in pigment or white. The majority of them 
are coast Fishes, rarely descending to a greater depth than 500 
fathoms, but some are pelagic. The Batoidei are a relatively 
modern race, first appearing towards the middle of the Mesozoic 
period, and evidently representing an assemblage of specialised 
Elasmobranchs adapted for a bottom-living existence. As re- 
marked by Smith Woodward, the three familes, Rhinobatidae, 
Raidae, and Trygonidae, are not so clearly differentiated before 
the close of the Cretaceous period as they subsequently become.’ 
The first two families, the Pristidae and the Rhinobatidae, are 
interesting connecting-links between such Selachii as the Rhinidae 
and the Pristiophoridae and the more specialised Batoidei hke the 
Skates, Rays, and Trygons. While they agree with the latter in 
the ventral position of the gill-clefts, the absence of an anal 
fin, and the caudal position of the dorsal fins, the body still 
retains an elongated and somewhat Shark-like shape, and shades 
off imperceptibly into a powerful swimming tail, and in the 
Pristidae at all events the pectoral fins are of moderate size 
and free from any fusion with the sides of the head. It must be 
admitted that the institution of the two sub-orders introduces 
a somewhat arbitrary distinction between certain families of 
Plagiostomes which has little to recommend it except custom 
and some measure of convenience. The two series of Fishes shade 
almost imperceptibly into one another, and the importance of 
the ventral position of the gill-clefts has probably been over- 
estimated. Primitively, the gill-clefts are lateral, and le wholly 
in front of the pectoral fins, a position which is retained in many 
! Vertebrate Palaeontology, Cambridge, 1898, p. 32. 
