JOURNAL, R.A.S. (CEYLON). [VOL. XI. 



with each different arrangement different families are at one 

 time brought into close connection and at another widely 

 separated. 



Alexander Agassiz, in speaking of the sea-urchins, says : — 

 "The sum of possible combinations is so great that it would 

 take no less than twenty years, at the rate of one new 

 combination a minute, for ten hours a day, to pass them in 

 review. We have not more than 2,300 species actually 

 representing for the Echini the results of these endless 

 combinations." 



Concerning the Principles on which the 

 following Tables are constructed. 



Linnaeus chose an easily-seized character on which to found 

 his main divisions. Thus, in classifying fish he arranged 

 them in orders according to the presence or absence and the 

 position of the ventral fins. When the ventrals were placed 

 forward on the throat, as in the cod, they were said to be 

 jugular ; when on the breast, as in the perch and great 

 majority of spinny-finned fishes, thoracic ; and when on the 

 belly, as in the carp, abdominal. In the eels they are 

 wanting, 



Cuvier, following Artedi, divided the bony fish into two 

 orders : the spiny (Acanthopterygii) and the soft-finned (Ma- 

 lacopterygii), according to the character of the vertical fins. 



Agassiz proposed another character, that of the scales, and 

 in this way divided the bony fish into two groups : those 

 with scales having a serrated edge and those with scales 

 having a smooth edge, Ctenoid and Cycloid. Now it is 

 evident that if the bony fish are arranged on any one of 

 these systems, the student, on consulting a list of them, 

 would at once have a definite piece of information ; if they 

 were arranged on the Linnaean system he would see at a 

 glance what fish had the ventrals in the same position ; if 

 on either of the other systems he would be equally able to 

 see in what fish the fins are spiny or soft, or the scales 

 ctenoid or cycloid. 



