324 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The apex of each triangular process is directed to the suture between two con- 

 tiguous radials to which it is attached just between the two adjacent apertures of 

 their central canals. Into these canals pass the secondary basal cords, one into 

 each of the two contiguous radials, so that one lies on each side of the interradial 

 process of the rosette. 



"As a general rule this process, both in Antedon and in Leptometra ("Antedon 

 phalangium") (figs. 428, 429, p. 321, and 589, 590, pi. 14), is short, triangular, and 

 slightly curved toward the ventral side. It is not always so, for I [Carpenter] have 

 frequently met with specimens of Antedon in which one or more of the interradial 

 processes of the rosette, after bending for a short distance toward the ventral side, 

 turns suddenly downward and extends toward the dorsal surface of the radial pen- 

 tagon. At the same time the parallel margins of each of these abnormally devel- 

 oped processes are so inflected toward the dorsal surface as to form a narrow 

 interradial spoutlike process. This is so applied to the projecting and similarly 

 inflected outer edges of the adjacent openings of the central canals in two contigu- 

 ous radials as to convert the interradial furrow lying between them into a com- 

 plete axial interradial canal, precisely similar in character to the radial axial canals." 



Carpenter met with one extreme case in which four of the five interradial 

 processes of the rosette were of this character. He states that this is the normal 

 condition of the interradial processes in the Comasteridse and in many of the endo- 

 cyclic forms. 



He continues: "Not only the interradial, but also the radial processes of the 

 rosette in Antedon may exhibit departures from their usual shape; for the removal 

 of the primary or dorsal layer at the salient angle of one or more of the five embryonic 

 basals may be incomplete so that the ends of the curved rays of the rosette exhibit 

 lateral processes which are the remains of the upper margins of the primitive basal 

 plates on which the radials rested. Occasionally the apex of the original basal is 

 left unabsorbed, so that the two lateral curved processes which remain after the 

 removal of the primary external layer along the median line of each plate remain in 

 connection with one another. * * * The triangular interradial process, which 

 is developed from a secondary calcareous deposit on the ventral side of the original 

 basal, has here become more or less completely united with these primary bars con- 

 necting the two lateral portions of the basal. The latter retain their primitive rela- 

 tion to the radials, for they remain united with them along the inner margin of their 

 dorsal faces; and as they partially cover in the secondary basal cords on their 

 dorsal aspect before they enter the central canals of the radials, I [Carpenter] will 

 call them the basal bridge." 



This basal bridge is a characteristic feature of the structure of the Comasteridse, 

 and of many of the other oligophreate comatulids, but is only rarely evident in 

 Antedon or in Leptometra. 



P. H. Carpenter says: "This tendency to an incomplete metamorphosis of the 

 embryonic basals of Antedon, and consequently to the abnormal persistence of a more 

 embryonic condition than usual, is of considerable interest, because in the Comas- 

 teridas and in many of the Oligophreata a basal bridge, representing the apex and 



