10 HAWAIIAN AND OTHER PACIFIC ECHIXI. 



single upright piece. This remarkable migration of the auricles is one of the 

 most notable features of clypeastroid morphology, and di\-ides the suborder into 

 two distinct groups. 



Another character of some importance is to be seen at the other end of the 

 interambulacra. Here, where the interambulacrum adjoins the genital plate, 

 there are usually two small plates to be seen side by side, terminating the two 

 columns of interambulacral plates, as in regular Echini. But in some clypeas- 

 troids, one of this pair greatly outstrips the other in growth and comes ultimately 

 to occupy the entire aboral end of the interambulacrum, increasing in size most 

 disproportionately as the test grows. In other clypeastroids, the oral end of the 

 interambulacrum may undergo a curious change during growth, by which the 

 primordial interambulacral plate remains in the basicoronal row, but is sepa- 

 rated from its fellows by the crowding in of ambulacral plates, making what is 

 called a "discontinuous interambulacrum." 



The shape of the test, the position of the anus, the numl)er and position of 

 the genital and madreporic pores, and the character of the buccal membrane are 

 all characters of more or less importance. Probabl}' the form of the test is very 

 •dii-ectly connected with the manner of life for it is known that the very flat, 

 discoidal species Uve a strictly subarenaceous hfe while the higher, more "bis- 

 cuit-shaped" species live on the bottom where they are only in part or not at all 

 covered by sand. It must be admitted that we know almost nothing from actual 

 observations as to the habits of these Echini, and possibly some assumptions as 

 to the significance of certain test-forms are quite erroneous. Accompanjang 

 the flattening of the test, there has been a marked development of internal 

 calcareous supports in the form of pillars, walls and horizontal floors. In some 

 cases, as Echinocyanus, these appear early in development and undergo little 

 change with age and are hence of systematic importance, but as a rule the deposit 

 of this extra calcareous matter goes on for a long time, perhaps thj'oughout life, 

 and with varying rates in different individuals. The resulting features of internal 

 structure are therefore very diversified and I have been unable to make any satis- 

 factory use of these characters for systematic purposes, except in the Fibulariidae. 



The occurrence of hmulcs or slits extending through the entire test, per- 

 mitting the free passage of water between the oral and aboral surfaces, is a 

 remarkable feature of manj- of the discoidal clypeastroids. The lunules maj' be 

 either ambulacral or interambulacral in position but alwaj's lie with the long 

 axis parallel to the radius of the test. They arise in the course of development, 

 either as notches in the test margin which become deeper with the growth of the 



