59 



umbilical sac becoming plaited, and iiiterdigitating with similar 

 folds of the walls of the uterus. 



The great majority of the Teleostei, or true fishes, deposit an 

 immense number of ova; but some, as the Embiotocidse, or vivip- 

 arous perch of this coast, are ovoviviparous, that is, the young are 

 hatched within the ovary. In these cases, however, there is no trace 

 of a jDlacenta developed, as in the Elasmobranchs. But there is 

 another and greater difference. In true fishes there is no union of 

 the sexes. The milt of the male is squeezed out over the ova of the 

 female — the two sexes, in some cases at least, assisting each other in 

 the operation by rubbing their bodies togetlier. But in the Sharks 

 and Rays, fertilization is secured in the same manner as in all ver- 

 tebrates above fishes, as Avell as in insects, Crustacea, spiders, etc., 

 namely, by the direct introduction of the male element into the 

 female reproductive organs. In this respect the Teleostei, however 

 specialized in other matters, took a step downwards, while the Elasmo- 

 branchs foreshadow, in their oviparous forms, the higher oviparous 

 vertebrates, and in their viviparous forms tlie mammalia. While 

 in the Teleostei the two sexes are usually much alike, and are distin- 

 guished externally only by slight differences in the form of the 

 abdomen, or in the color (especially at the breeding season), the 

 males of the Elasmobranchs may be readily distinguished by the 

 pair of large organs known as " claspers," which are really intromit- 

 tent organs. 



The skin of the members of this class is more or less studded with 

 calcified papillae, forming, when the papillse are numerous and 

 thickly set, what is known as shagreen; and the entire skeleton is 

 cartilaginous. 



This class is divided into two orders, the first of which includes 

 only the Chimsera and its allies, which are characterized by the 

 coalescence with the skull of the cartilage forming the upper jaw and 

 palate, and by the presence of a single gill-opening only, and the 

 Plagiostomi, or Sharks and Rays, in which the jaws are distinct 

 from the skull, and there are from five to seven gill-openings. The 

 pouches within these branchial slits are narrow, and divided from 

 each other by a membrane, but the respiratory processes do not 

 extend to their edges, except in Chimsera and its allies. 



The Sharks and Rays, the two sub-orders of the Plagiostomi, are 

 distinguished from each other chiefly by tlie more or less cylindrical 

 form and lateral gill-openings of the former, and the depressed body 

 and ventrally situated gill-openings of the latter. But the two orders 

 approach closely by such forms as the depressed monkfish and the 

 sawfish {Pristiophorus), with lateral gill-openings, and the scarcely 

 more depressed sawfish (Pristls), with gill-openings on the under 

 surface. 



ORDER HOLOCEPHALI — CHIMERAS. 



Chimera collici, Rat-tail, Rat-fish — This species, the Pacific rep- 

 resentative of the Chimsera m.onstruosa of the Atlantic, is by no 

 means rare on all parts of the North American coast north of Point 

 Concepcion, and is occasionally, on account of its bizarre appear- 

 ance, brought into the market of San Francisco as a curiosity. 

 In museums it is one of the most ordinarily occurring species, for 

 every novice in icthyology who procures one, believes that such 

 thing was never seen before, and forwards it accordingly. 



