120 THE ANATOMY OF VERTEBRATED ANIMALS. 



The eggs are very large, and comparatively few. 



The Dogfishes, the Rays, and the Chimmra^ are o\-iparouSj 

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

 viviparous, and, in certain species of 31ustelvs {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 

 pcrijohery 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 Elasinobrmichii are divided into two groups, the Uolo- 

 cephali and the Plagiostomi, 



In the Holocephali^ the palato-quadrate and suspensorial 

 cartilao^es 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 difi'er in 

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

 tains the living ChimcBra and Ccdlorhynehus, the extinct 

 Mesozoic JEdaphodon 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 skull. The branchial clefts are not covered by any oper- 

 cular membrane. The teeth are usuallv numerous. 



The Plagivstonii are again subdivided into the Sharks 

 [Selachii or Squcdi), 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 (Iiajce), 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 PJlasmohranchii 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 Plagiostomi occur in the Mesozoic 

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

 ILlasmohranchii abound in the Permian and Carboniferous 



