A MONOGRAPH OF THE EXISTING CEINOIDS 351 



closer, so that the ultimate result is almost a close suture, the joint line appearing 

 externally as a uniform fine straight line and the joint faces showing scarcely more 

 than a trace of the typical synarthrial sculpture (vol. 1, pt. 1, fig. 36, p. 75) which 

 may be confined to a relatively small area, while, on the other hand, they may develop 

 radiating ridges, so as gradually to become transformed into an almost perfect syzygy 

 (vol. 1, pt. 1, fig. 39, p. 75). As a rule the extremely close synarthry is characteristic 

 of the slender-armed form and the syzygy of the stout-armed type. All forms of 

 intermediates between these two types of articulation occur. 



Dimorphism. There are two forms of dimorphism in this species which are 

 not correlated with each other. 



In some individuals the arms are long and slender, tapering gradually and 

 evenly from the base to the delicate tip, while in others they are short and very stout, 

 broadening from the base to about the fourteenth brachial and from that point 

 tapering rapidly distally. The slender and stout forms are very different in appear- 

 ance, but intergrades between the two are not infrequent. Very commonly the 

 posterior ungrooved arms are much shorter and stouter than the grooved anterior 

 arms. (See vol. 1, pt. 2, fig. 163, p. 86.) 



As a rule, specimens in which the calices are of the same size will have arms of the 

 same length. More or less frequently, however, an individual is found in which the 

 distal portion of the arm instead of being moderately stout and tapering evenly 

 to the tip is attenuated and greatly elongated, with very long brachials and widely 

 separated pinnules, so that it resembles more or less closely the distal portion of the 

 arms in Antedon. 



This curious prolongation and attenutation of the distal portion of the arms 

 occurs in both the slender and the stout-armed forms. 



Both types of dimorphism occur in the same way in Comactinia echinoptera. 

 But in the latter species they seem to show a definite relation to environmental 

 conditions which is scarcely the case in Comatula pectinata. 



Abnormal specimens. The specimen from Java in the Copenhagen Museum 

 bearing the name Actinometra affinis Liitken, MS., has 12 arms. 



Individuals with 11 arms have been recorded from Siboga station 99 (1), Singa- 

 pore (8), the Philippines (1), and Holothuria Bank in 62.1 meters (1). 

 A detached arm from Singapore shows distal arm branching. 

 Commensalism. One of the specimens from Singapore has a small Ophiomaza 

 with the disk 4 mm. in diameter covering the ventral surface of the disk. This 

 ophiuran (see vol. 1, pt. 2, p. 643) is common at Singapore, but has not previously 

 been reported in connection with this crinoid. 



Geographical variation. The stout-armed form of this species predominates and 

 reaches an extreme development in the eastern portion of its range, from the 

 Philippine Islands southward to and including New Guinea and Australia. West- 

 ward the slender-armed form becomes more common and the stout-armed form less 

 and less extreme, so that in the Java Sea the slender armed form greatly predominates, 

 and the few individuals which may be said to represent the stout-armed type have 

 the characteristic features only moderately developed, and commonly confined to 

 the ungrooved posterior arms. 



