266 



FISHES 



imperfect trtmcated cones with their apices directed backwards, 

 the successive cones adhering so closely to one another that they 

 combine to form a central conical chamber with a spirally disposed 

 cavity winding round it. In D, on the contrary, the free edge 

 of the valve is deflected forwards, so that, as in C, a nest of cones 



Fig. 159. — Examples of various types of the spiral valve in Elasmobranchs. A, B, C, 

 and D in specimens oi Raia spp. ; E, in Sphyrna malleus. A, B, and D represents 

 longitudinal sections of tlie intestine, the ventral portion of the valve being re- 

 moved. In C successive portions of the ventral wall of the intestine have been cut 

 out. In E the intestine has been opened along the mid-ventral line and its wall 

 reflected to the right and left ; the ventral portion of each coil of the " scroll " valve 

 has been removed. In most of the figures the pylorus is .shown in the upper part, 

 and the "rectal" gland in the lower. (From T. Jeffery Parker.) 



is formed, but the apices of the successive cones are directed 

 forwards instead of backwards. Notwithstanding these variations 

 in the structure of the valve as a whole, the first coil or half coil 

 nearly always resembles that described in A. 



It is obvious that the structure of the valve varies consider- 

 ably within the limits of the genus, and it may be added that 

 various intermediate types of structure occur between A and B, 



