Simpson — A Revision of the Gorgonellidae. 23 



only character which may with certainty be used as a basis of classification 

 is the number of lo)igitiuli)ial scries as defined by the number of large main 

 canals. 



(4) Axis. 



The axis consists of a horny substance whose chemical composition has 

 never been thoroughly investigated, and which is insoluble in the more 

 common organic solvents. 



This is impregnated with carbonate of lime. 



The axis is deposited in the form of thin concentric laminae, so that a 

 cross-section (fig. 3) shows annular markings. These are more densely 

 calcareous towards the centre and appear whiter, so that the axis has often 

 been described as having a calcareous core. This is not actually the case, 

 howevet, as the horny material exists even in the very innermost layers. In 

 the younger parts of the colony there is very little lime deposited, so that it 

 is softer and more flexible. 



It is noteworthy, however, that even in colonies attaining a height of 

 6 feet or more the axis is flexible almost to the very base. Near the base, 

 however, it is very hard, and is cut witli a knife only with difficulty. The 

 actual hardness varies in different specimens of the same length. 



There are small ridges and furrows on the surface (fig. 3) which give the 

 cross-section a serrated outline. These, as has been already explained, 

 correspond to the canals of the inner longitudinal series and diminish in 

 number from the base upwards (fig. 11, a, b, and c). 



Tiie colour varies in the ditferent colonics ; but, as a rule, it is olive-green 

 towards the base, passing to pale yellow near the tip. In some specimens, 

 however, it is almost white throughout, due in great part to a larger 

 deposition of liuu>, and consequently, in the.se, the axis is less flexible and 

 more brittle. 



The increase in the thickness of the older part of a colony is due, not to 

 an increase in the thickness of the coenenchyma, but almost entirely to an 

 increase in the thickness of the axis (see tigs. 10, 13, 10, and 43). 



(5) Spicules. 



The spicules of this group are characterized by their exlrcme sinallness ; 

 in fact, in no other group of Alcyonaria do we find the prednniiiuinl spicule 

 so minute. Measurements of these with any precision are only possilile 

 with a high magnification. 



The largest measurements for the group, viz., those in Nicclln dichotoma, 

 are only 0'2o mm. x 0*0() mm. ; but in the genus Juncella the largest are 

 those in Juncella trilineata, which are over 0'070 mm. x 0'038 mm. ; while in 



