342 FISHES. 



near the angle of the disk, and frequently also the sides 

 of the head. In species which are armed with bucklers or 

 asperities it is the female which is principally provided with 

 these dermal productions, the male being entirely or nearly 

 smooth. Also the colour is frequently different in the two sexes. 

 Other genera of this family are Psammobatis, Sympterygia, 

 and Platyrhina. Although probably this family was well re- 

 presented in cretaceous and tertiary formations, the remains 

 found hitherto are comparatively few. Arfhropfencs, from the 

 Lias, seems to have been a true Eay ; and dermal spines of a 

 species allied to the Thornback {Baja antiqua) are abundant 

 in the crag deposits of Suffolk and ISTorfolk. 



Fifth Family — Tkygonid^. 



The pectoral fins are uninterruptedly continued to, and con- 

 fluent at, the extremity of the snout. Tail long and slender, 

 without lateral longitudinal folds ; vertical fins none, or im- 

 perfectly developed, often replaced by a strong serrated spine. 



The " Sting-Eays " are as numerous as the Eays proper, 

 but they inhabit rather tropical than temperate seas. The 

 species armed with a spine use it as a weapon of defence, 

 and the wounds inflicted by it are, to man, extremely painful, 

 and have frequently occasioned the loss of a limb. We have 

 mentioned above (p. 190) that the danger arises from the 

 lacerated nature of the wound as well as from the poisonous 

 property of the mucus inoculated. The spines (Fig. 98, p. 

 190) are always barbed on the sides, and may be eight or nine 

 inches long in the larger species. They are shed from time 

 to time, and replaced by others growing behind the one in 

 function, as the teeth of the fishes of this order, or as the 

 fangs of a poisonous snake. Fossil species of Trygon and 

 Urolophus occur in the tertiary strata of Monte Bolca and 

 Monte Postale. The genera into which the various species 

 have been divided are. the following ; — 



