THE BLUE SHARK AND ITS ALLIES 2897 



Spiracles are frequently developed on the upper surface of the head; and the inter- 

 calary cartilages already alluded to are ovoid or diamond-shaped structures occurring 

 between the superior arches of the vertebrae. The eggs are generally invested in 

 horny rhomboidal capsules, furnished at the four corners with long tendril-like 

 filaments, by which they attach themselves to the stems of seaweeds and other 

 bodies, as shown in the figure of the lesser spotted dogfish given on p. 2904. In 

 some species, however, the eggs are hatched within the body of the female; and in 

 all cases the embryos are furnished with external gills, which are shed before birth. 

 All the members of the order subsist on animal substances, but whereas the typical 

 sharks are highly predaceous creatures, seizing and devouring everything they come 

 across, some of the largest species are armed only with small teeth, and feed on 

 mollusks and other invertebrates. The rays, too, are largely shellfish eaters, and 

 most of them differ from the sharks in living on or near the bottom, instead of 

 swimming about actively at or just below the surface. All the species are typically 

 marine, but many ascend tidal rivers, and in the Viti L,evu Lake in Fiji, as well as 

 in the Nicaragua Lake in South America, there are sharks dwelling permanently in 

 fresh water. The species inhabiting the former lake, which is cut off from the sea 

 by a cataract, is Carcharias gangeticus, common alike in the Ganges and in the 

 Tigris, and ascending in the latter river to a distance of three hundred and fifty 

 miles from the sea in a straight line. Then, again, a species of sawfish is found in 

 a fresh-water lake in the Philippines. It has been commonly stated that sharks 

 have the power of scenting their prey from a distance, since they rapidly congregate 

 whenever animal refuse or other decomposing matter is cast overboard from a ship; 

 but it may be suggested that such assemblages, as in the case of vultures, are rather 

 due to one shark following the movements of another, and thus being attracted to 

 the central point. The order was formerly divided into two subordinal groups, 

 based upon the conformation of the body; the one group including all the sharks 

 and dogfishes, and the other the rays and their immediate allies. It has been found, 

 however, that although this difference in bodily form is of considerable importance 

 in classification, yet that it does not constitute the essential line of distinction, which 

 is based upon a difference in the internal structure of the bodies of the vertebrae. 

 Taking this characteristic as a basis, the members of the order may be arranged in 

 two subordinal groups, the first of which comprises the true sharks and dogfishes, 

 while the second includes the spiny dogfishes, sawfishes, eagle rays, and rays. 



THE BLUE SHARK AND ITS ALLIES Family CARCHARUDsE 



The well-known blue shark (Carcharias glaucus}, of which examples are 

 depicted in our colored plate, may be taken as the typical representative of the 

 leading family of the first suborder'. Before, however, indicating the character- 

 istics of the family, we must refer to those of the suborder, for which the name 

 of Asterospondyli has been sugge.sted. The essential feature of this group is to be 

 found in the circumstance that when the bodies of the vertebrae are fully calcified, 

 the radiating plates in the interior predominate over the circular ones, so that a 

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