DOEOOIDARIS PANAMENSIS. 21 



the secondary ones of the interambulacral areas (PI. 3, fig. 5). This row 

 of primary ambulacral tubercles is flanked by an inner irregular row of 

 small miliaries, single near the actinal and abactinal extremities and 

 double along the equatorial region of the test. The primary interam- 

 bulacral tubercles are surrounded by a row of secondaries placed close to 

 the scrobicular area ; the rest of the primary plate is closely packed by 

 smaller secondaries and miliaries (Pis. 3, fig. 6; 4, figs. 6, 7). 



The structure of new interambulacral plates is shown in PI. 4, figs. 1, ..'. 

 In fig. 1 the eighth primary plate of the left interambulacral area is trian- 

 gular, and has only a single rudimentary primary tubercle, showing the 

 radiating and circular arrangement of the limestone cells of the plate. In 

 fig. £ the eighth primary plate of the odd interambulacrum has assumed a 

 pentagonal shape. In addition to the rudimentary primary tubercle it has 

 three fairly developed secondary tubercles and one less advanced. The 

 arrangement of the cells of the plate is more irregular than that of the 

 younger plate ; their arrangement in radiating ridges is better seen in an 

 older plate (PI. 4, fig. o), the sixth from the actinostome. 



Considerable changes take place in the abactinal system during growth. 

 In a small specimen measuring 10 mm. in diameter (PI. 3, fig. i), the 

 genital and ocular plates as well as the outlines of the anal system are 

 marked for their sharp angular outlines. In a somewhat older specimen, 

 18 mm. in diameter (PI. 3, fig. 2), the angles of these plates have become 

 slightly rounded, and in a still larger specimen, 30 mm. in diameter (PI. 3, 

 fig. J), the sides of the genital plates are convex or concave, with rounded 

 points. The dividing lines between the genital plates are greatly reduced 

 in length, the cusps of the anal system and of the ocular plates almost 

 connecting. In young specimens (PI. 3, fig. 1) only a few verrucas are 

 found on the plates of the abactinal system. In somewhat older specimens 

 they are found only on the anal edge of the genital plates ; the rest of 

 the plate is covered by small secondary tubercles and miliaries. In still 

 older specimens (PI. 3, fig. 3) both genital and ocular plates are covered by a 

 regular granulation of small secondary tubercles, with a few miliaries in the 

 central part of the plates. The genital pores are not pierced in the 

 smallest specimen figured (PI. 3, fig. 1). They appear in older stages, and 

 are well developed in a specimen 18 mm. in diameter (PI. 3, fig. ;.'); they 

 are fully as well marked as in a still older specimen (PI. 3, fig. 3). 



A number of madreporic pores are found in a specimen of 10 mm. in 



