136 THE ANATOMY OF VEKTEBEATED ANIMALS. 



are true Fallopian tubes, which communicate freely with 

 the abdominal cavity at their proximal ends. DistaUy, they 

 dilate into uteiine chambers, which unite and open into the 

 cloaca. 



The eggs are very large, and comparatively few. 



The Dogfishes, the Rays, and the Chimoera are ovi- 

 pai'ous, and lay eggs enclosed in hard leathery cases : the 

 others are viviparoiis, and in certain species of Mustehtu 

 ikevis) and Carcharias, 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 embi-yos of most Elasmobranchs are, at first, pro- 

 \-ided 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 function- 

 ally replaced by internal gills as development advances. 



The Elasmobranchii are divided into two gi'oups, the 

 HolocephaU and the Plagiostomi. 



In the HolocephaU the palato-quadrate and suspensorial 

 cartilages are united with one another and with the skuU 

 into a continuous cartilaginous plate ; the branchial clefts 

 are covered 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 diifer in structure from those of the Plagiostomi. This 

 suborder contains the living Chimcera and CaUorhynchus. 

 the extinct Mesozoic Edapliodon and Passalodon ; and, very 

 probably, some of the more ancient Elasmobranchs, the 

 teeth of which are so abundant in the Carboniferous lime- 

 stones. ' 



In the Plagiostomi, the palato-quadrate and suspensorial 

 cartilages are distinct from one another, and ai-e moveable 

 upon the skull. The branchial clefts are not covered by 

 any opercular membrane. The teeth are usually niimerous. 



The Plagiostomi are again subdivided into the Sharks 

 iSelachii or Squall), with the branchial apertiu-es at the sides 



