474 Prof. J. C. Ewart. [Feb. 25, 



our tabulation areas. One main object is that our primary frame- 

 work of areas arid sub-areas should separate our species and genera 

 (so far as possible) into those areas and sub-areas; if a boundary 

 line is drawn between two sub-areas A and B, so that nearly all the 

 species found on one side of it are also found on the other, then we 

 might as well, in this tabulation, have thrown the two sub-areas A and 

 B into one, and saved ourselves labour. This brings us round prac- 

 tically pretty nearly to Wallace's view again ; i.e., geographic frame- 

 work for reference and tabulation must be as near as possible to a 

 system of natural biologic regions, subject to the condition that the 

 boundary lines are rapidly and accurately fixed, and are easily re- 

 membered. It is impracticable to effect large tabulations of tens of 

 thousands of specimens if it is necessary continually to refer to 

 some special large-scale map. 



The present paper is not intended to include marine regions or 

 areas. 



V. ' ; The Electric Organ of the Skate : Observations on the 

 Structure, Relations, Progressive Development, and Growth 

 of the Electric Organ of the Skate." By J. C. EWART, 

 M.D., Regius Professor of Natural History, University of 

 Edinburgh. Communicated by Prof. J. BuRDON SANDERSON, 

 F.R.S. Received February 10, 1892. 

 (Abstract.) 



After referring to the observations of Stark, the discoverer of the 

 skate's electric organ, and to the work of Robin, Leydig, Babuchin, 

 and others, the author describes the arrangement of the muscles in 

 the tail of Selachians with a view to determining which muscles in 

 the skate are transformed into the electric organs. 



By comparing the caudal muscles of Scy Ilium, Lamna, Myliobatis, 

 and Raid, it is made out that, while the middle row of muscular cones 

 remains unaltered in the sharks and rays, it is transformed into a 

 more or less perfect electric organ in the skates, the various members 

 of the genus Raia. It is pointed out that, while the middle row of 

 muscular cones is transformed in Eaia into electric cones, the two 

 adjacent rows of cones as in the rays and certain sharks diminish in 

 size, and in some cases disappear about the middle of the tail. 



In considering the structure of the organ, it is stated that, when 

 the various modifications are taken into consideration, it may be 

 described as consisting of a series of electric cones made up of more 

 or less completely metamorphosed muscular fibres. Twenty-eight 

 distinct cones were counted in the organ of JR. batis. The first, 

 which in a half grown fish measured 5 cm. in length, was all but 



