AR/EOSOMA BICOLOR. 179 



Arseosoma bicolor a. Ag. and ci. 



Aathenosoma bicolor A. Agassiz and Clark, 1907. Bull. M. C. Z., LI, p. 118. 



Plates 64, figs. 1-8; 71; 72. 



This species, of which only a single specimen was collected, is nearly 

 related to Oivstoni, but differs in color and in certain features of the 

 test. The coronal plates are low and very numerous, 44 in the inter- 

 ambulacra and 75 in the ambulacra (Pis. 71, 72, figs. 3, 4) ; in Oivstoni 

 of the same size (125 mm.) the numbers are 38 and 60 respectively. 

 The test is more flexible abactinally than in Owstoni, and the bare median 

 ambulacral and interambulacral areas are more marked (PI. 71, figs. 1, 2). 

 The test and spines are dull yellowish actinally, while on the abactinal 

 surface the interambulacra are chiefly yellow and the ambulacra are dull 

 violet. These colors are not sharply defined, but contrast with each other 

 nevertheless. On the actinal side the primary ambulacral tubercles form 

 two median longitudinal series (PL 71, fig. 2). At the ambitus there are 

 two additional series of primaries somewhat smaller which extend irregularly 

 along the ambulacrum for about two-thirds the distance from the ambitus to 

 the apex (PI. 71, fig. 1). The actinal primary interambulacral tubercles are 

 arranged in six very irregular rows ; in the outer rows, adjoining the ambu- 

 lacral area, they are closely packed (PI. 71, fig. 2) two-thirds of the distance 

 from peristome to ambitus. On the abactinal surface the primary interam- 

 bulacral tubercles extend in irregular open rows almost to the apical system 

 (PI. 71, fig. 1). The rest of the plate is closely covered with miliarie.s (PI. 72, 

 fig. Jf). The primordial actinal interambulacral plate is large and prominent 

 (PI. 72, fig. l). The imbricating actinal plates, in prolongation of the ambu- 

 lacral series, cover the whole buccal membrane except at the actinal margin 

 of the interambulacra, where there are a few minute plates (PI. 72, fig. l). The 

 actinal plates are covered with minute secondaries and miliaries arranged in 

 horizontal rows. 



The genital plates in bicolor are not so elongated as in Owsioni, for they 

 separate only the first pair of interambulacral plates and touch the second 

 (PI. 72, fig. 2), while in Owstoni they separate the first two pairs and touch, 

 sometimes nearly separating, the third. In bicolor four of the genital plates 

 are remarkable in that the outer part of the plate (i. e. the part distal to the 

 pore) is separated by a regular suture from the remainder of the genital and 



