120 THE ANATOMY OF VEUTEBRATED ANIMALS. 



The eggs are very large, and comparatively few. 



The Dogfishes, the Rays, and the Chinimra, are oviparous, 

 and lay eggs, enclosed in hard, leathery cases ; the others are 

 viviparous, and, in certain species of Musteliis (icevis) and Car- 

 charias, a rudimentary placenta is formed, the vascular walls 

 of the umbilical sac becoming plaited, and interdigitating with 

 similar folds of the wall of the uterus. 



The embryos of most Elasmobranchs are, at first, provided 

 with long external branchial filaments, which proceed from the 

 periphery of the spiracle, as well as from most of the branchial 

 arches. These disappear, and are functionally replaced by 

 internal gills as development advances. 



The Elasmohranchii are divided into two groups, the Holo- 

 cephali and the Plagiostomi. 



In the Solocephali, the palato-quadrate and suspensorial 

 cartilages are united with one another and with the skull into 

 a continuous cartilaginous plate ; the branchial clefts are cov- 

 ered by an opercular membrane. The teeth are very few in 

 number (not more than six, four of which are in the upper, 

 and two in the lower jaw, in the living species), and differ in 

 structure from those of the Plagiostomi. This sub-order con- 

 tains the living Ghimaera and Gallorhynchus, the extinct 

 Mesozoic Edaphodon and Passalodon ; and, very probably, 

 some of the more ancient Elasmobranchs, the teeth of which 

 are so abundant in the Carboniferous limestones. 



In the Plagiostomi, the palato-quadrate and suspensorial 

 cartilages are distinct from one another, and are movable upon 

 the skuU. The branchial clefts are not covered bj- any oper- 

 cular membrane. The teeth are usually numerous. 



The Plagiostomi are again subdivided into the Sharks 

 (Selachii or Squali), with the branchial apertures at the sides 

 of the body, the anterior ends of the pectoral fins not connected 

 with the skull by cartilages, and the skull with a median facet 

 for the first vertebra ; and the Rays (Pajce), with the branchial 

 clefts on the under-surface of the body, the pectoral fins united 

 by cartilages to the skull, and no median articular facet upon 

 the occiput for the first vertebra. 



The Elasmobranchii are essentially marine in their habits ; 

 though Sharks are said to occur very high up in some of the 

 great rivers of South America. 



Both divisions of the Plagiostom,i occur in the Mesozoic 

 rocks. In the Palasozoio epoch, dermal defences and teeth of 

 Elasm^ohranchii abound in the Permian and Carboniferous 



