46 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



segments which are strongly constricted centrally are found in very small species, and 

 in medium sized species from deep water. 



The distal ends of the cirrus segments may be unmodified, or they may be produced 

 and overlap the bases of those succeeding. The earlier longer segments are usually 

 from two to three times as long as broad ; but they may be as much as six times as long 

 as broad. The distal segments are never broader than long, but very rarely are more 

 than twice as long as broad. 



The longer earlier segments are always more or less constricted centrally so that 

 the ventral, and usually also the dorsal, profile is more or less concave, and the width 

 of the ends in lateral view may be as much as twice that of the central portion or even 

 more, though usually it is less. 



The dorsal surface of the segments is always rounded, never carinate. 



The distal edges of the radials in the median line are usually even with the run of 

 the centrodorsal which may, however, extend beyond them. Interradially the distal 

 angles of the radials are never produced, being visible as low and often very narrow 

 triangles. 



Occasionally (as in Antedon duebeni, Annametra occidentalis, sometimes in Antedon 

 bifida, etc.) there are prominent groups of perisomic interradials between the IBr 

 series. 



The disk, which is somewhat, though never strongly, concave in the interradial 

 areas, is naked or, rarely in the larger species, bears a greater or lesser number of 

 calcareous concretions. 



Sacculi are always abundant and conspicuous, regularly distributed along the sides 

 of the ambulacral grooves. 



The IBr series and arm bases in this group show but little variation. The IB^ are 

 always narrow with converging sides which may, however, be extended laterally to meet 

 those of their neighbors by a development of the ventrolateral border which often bears 

 a tubercle at the anterolateral angle. The axillaries are always broad, usually as broad 

 as long or broader than long, always with the median portion of the proximal border 

 more or less produced downward at the expense of the IBr^ with all the sides, especially 

 the two distal, concave, always rising to a more or less prominent synarthrial tubercle 

 with the IBri and, if the ventrolateral borders of the latter are produced, with a 

 ventrolateral production beneath the lateral angles to match, which frequently bears 

 a tubercle. 



There is never any spinosity, carination, or other ornamentation on the dorsal 

 surface of the IBr series, though the synarthrial tubercles, when strongly developed, 

 may be roundedly carinate. The development of spines on the edges of these ossicles 

 is rare. 



The first two brachials reduplicate the essential features of the elements of the IBr 

 series, and the second is always much larger than the first and irregularly quadrate in 

 form. 



In this group the division series and arm bases always lie in planes making a very 

 large angle with the dorsoventral axis, in some cases almost a right angle, so that when 

 the arms are fully extended the animal is almost flat. 



The arms vary in adult individuals from 15 to 200 mm. in length very nearly 

 this whole range is covered by Antedon bifida, alone but are usually between 20 and 



