30G CLYPEASTER SUBDEPRESSUS. 



ECHINANTHIDAE. 



Subfamily Echinanthidae A. AGASB. 



CLYPEASTER. 



Clypeaster Lamk. 181 G. An. s. Vert, (emend.) 



The flat Clypeastroids arc readily enough distinguished externally from 

 their allies, yet their generic distinction would often be difficult, if not 

 impossible, with such species as Echinanthus testudinarius from Australia, 

 were it not for the flatter character of actinal surface, the less sunken mouth, 

 and the better defined ambulacral grooves radiating from the actinostome. 

 But the interior at once furnishes us a set of characters of great impor- 

 tance, in the absence of the double ambulacral chamber, and the slender 

 needle-like pillars which replace the massive columns of the true Echinan- 

 thus. Actinal surface flat ; actinostome sunken in a cavity of small extent) 

 and well circumscribed, while in the other genus the cavity commences 

 quite gradually. 

 Clypeaster subdepressus 



! Echinanthus subdepressus GRAY, 1825. Ann. Phil. 

 ! Clypeaster subdepressus Agass. 1836. Prod. 



PL XP; PL XI'. f. 1, .' ; PI. XII*. f. 4 ; PL XTTI. f. 10-18. 



The onlv specific differences thus far noticed in the species of this genus 

 are the character of the tuberculatum, the position of the genital openings, 

 and the comparative width of the median ambulacral space in the petals. 

 The outline is elliptical, with slightly re-entering sides in the median intoram- 

 bulacral spaces. The greatest breadth is usually opposite the termination of 

 the posterior pair of ambulacra ; but in specimens in which the edge of the test 

 becomes swollen, the greatest breadth is opposite the anterior pair of ambu- 

 lacra (/'/. XI'. f. 1,2). The test usually is nearly flat from the margin to 

 the extremity of the ambulacral petals, then it commences to rise, and rises 

 quite suddenly, arching regularly to the abactinal pole. The odd petal and 

 the posterior pair have the same width of median ambulacral space ; in 

 the anterior pair this median space is narrower; they are also much shorter 

 than other petals, though the posterior pair are longer than the odd anterior 

 one. The genital openings are small, placed close to the madreporic body. 

 The general outline of the petals is somewhat lanceolate, but this varies 



