84 A Guide to the Zoological Collcctiotis 



placed spines— these being more numerous in the female. 

 The Rays live on the sea-bottom and progress by the 

 undulations of their pectoral fins. The genera that are 

 found in Indian Seas are Raja and Plalyrhina. The 

 Indian species of Raja, three in number, are only found 

 in deep water. 



Family V. Trygonidce [Case 34]. 



The Sting-rays have a disk-like body and a long lash- 

 like tail, which is even more sharply demarcated from 

 the disk than is that of the true Rays. Moreover, near 

 the base of the tail, in many species, there exists a great 

 erectile barbed spine, which is capable of inflicting a 

 severe and even dangerous wound. The spine is shed 

 and replaced by reserve spines, like the poison fang of 

 a snake. The Sting-rays, like the true Rays and Electric 

 Rays, live on the bottom : they have pavement-like 

 crushing-teeth for feeding on hard-shelled mollusks and 

 crustaceans. They are particularly numerous in the 

 muddy water at the mouths of estuaries, and, like so 

 many fishes that have this habitat, they have a tendency 

 to spread up the estuaries into freshwater and so, in some 

 cases, to become incorporated in the freshwater fauna. 

 Many. Try gons bring forth their young alive, the embryo 

 being nourished during its development, not by a vas- 

 cular placenta, but by a milk-like secretion which is 

 poured forth from a multitude of special glands in the 

 uterine mucous membrane and finds its way into the gullet 

 of the embryo through the latter's spiracles. Speci- 

 mens in Case 12 illustrate this singular method of intra- 

 uterine nutrition. Two species of Trygon, T. bennettii 

 and T. pastinaca, are common to the Atlantic and Indo- 

 Pacific, the latter also being found in the Mediterranean. 

 Pteroplatcea also is found both in the Atlantic and in the 



