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, Molluscs, 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 families, Ehinobatidae, 

 Eaiidae, and Trygonidae, are not so clearly differentiated before 

 the close of the Cretaceous period as they subsequently become. 1 



The first two families, the Pristidae and the Rhinobatidae, are 

 interesting connecting-links between such Selachii as the Ehinidae 

 .and the Pristiophoridae and the more specialised Batoidei like the 

 Skates, Bays, 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 lie wholly 

 in front of the pectoral fins, a position which is retained in many 



1 Vertebrate Palaeontology, Cambridge, 1898, p. 32. 



