334 PLAGIOSTOMATA. 



Family VIII RAIIDJE. 



Disk broad and depressed, of a rhombic or circular form : tubercles or spines 

 usually present. The pectoral fins extend to the snout. Tail with a longitudinal 

 skinny fold along either side. No serrated caudal spine. Electric organs absent.* 



The forms of rays and skates existing in the seas around our coasts consist of 

 species which appear to be more or less sedentary ground-feeders, carnivorous 

 and very voracious in their habits, and although mostly consuming molluscs and 

 Crustacea, devour likewise large quantities of fish. 



Geographical distribution. They are found in the seas of both hemispheres, 

 but are more numerous in the northern than in the southern, while they are 

 rarely taken far from the coast unless off banks in the ocean. 



. Genus I Raia, sp. Artedi. 



TJraptera, Muller and Henle. 



Tail distinct from the dish. Teeth pointed or obtuse. Nasal valves united in the 

 median line where no interior free edge exists. Pectoral fins surround the outer 

 margin of the dish, but are not extended on to the front edge of the snout. Two 

 spineless dorsal fins situated on the tail. End of tail with or without a rudimentary 

 caudal fin. 



These fishes having the gill-openings, as well as the mouth, on the lower 

 surface of the body, and generally lying at the bottom of the sea, require the 

 assistance of breathing holes or spiracles at the top of the head, for the purpose of 

 supplying the gills with water for respiration ; these are placed near the hind edge 

 of the orbit, are of large size, and can be closed at will. Doubtless living at the 

 bottom of the sea these fishes on rising to the upper waters or coming into shallower 

 places might find the stimulus of light too much, were it not that they 

 possess a modification of the iris, which by means of a veil or curtain 

 which has been compared to a vine-leaf, surrounded by 12 digitations, can 

 be expanded or contracted as required, and prevents an excess 



of light obtaining an entrance ^tBfjIKt^. through the pupil. This would 

 be especially serviceable to these Jr^ __ * fishes during the summer and 

 autumn months when they come k, \^fct$p] > to the surface and bask in the 

 sun. They progress by an un- ^^L^r-^J^ dulating motion of the pectoral 

 fins, as seen in pleuronectoid fishes as soles. 



EYE OF RAIA MACULATA. 



Montagu in 1815 (Mem. Wern. Soc.) drew attention to the differences which 

 were present, in accordance with their sex, in some of these fishes. He remarked 

 that the males were to be distinguished by their genitals being of a most 

 curious structure, pendant one on either, side of the tail, between it and 

 the anal fin (see page 329). Another sexual character which he pointed 

 out is to be found in the teeth of the sexes, and he described the blunt 



* See article on " the existence of an electric apparatus in the flapper skate and other rays." 

 Stark, Froc. Hoy. Soc. Edinburgh, 1844-50, p. 1. 



