The axis in this sub-order may be horny, or horny with a core of calcium carbonate, or composed of horn impregnated with calcium carbonate, or of nodes of horn alternating with internodes of calcium carbonate. It may be distinguished from the axis of the Pseudaxonia by the fact that in no case have definite spicules been observed to take part in its formation. It has been suggested that as the Axifera represent a line of descent distinct from that of the Pseudaxonia they should be placed in a separate order. Apart from the character of the axis, however, the two sub-orders show so many affinities in their general anatomy that it is better to regard the two lines of descent as united within the Gorgonacean limit. It is very improbable that the two groups sprang independently from a stoloniferous ancestor. FAM. 1. ISIDAE.—This family includes all those Axifera in which the axis is composed of alternate nodes of horn and internodes of calcareous substance. There can be little doubt of the close affinities of many of the genera of this family with the Melitodidae among the Pseudaxonia. In both the coenenchym is thin and the coelenteric cavities short. No important differences have been observed between the structure of the zooids of the two families, and now that we know that the "nutritive canals" of _Melitodes_ do not perforate the nodes there is no important difference left between the coenosarcal canal systems. The structure and method of calcification of the internodes of the two families are very similar. The main difference between them is that the nodes of the Isidae are purely horny, whereas in the Melitodidae the horny substance of the nodes contains calcareous spicules. The principal genera are:—_Isis_, Linnaeus; Pacific Ocean. This genus forms substantial fan-shaped colonies with, relatively, a thick coenenchym, short stout internodes and black horny nodes. _Mopsea_, Lamouroux; Coast of Australia. The verrucae are club-shaped and are arranged in spiral rows round the stem. _Acanella_, Gray; principally found in deep water in the Atlantic Ocean but also in the Pacific. The internodes are long and the branches arise from the nodes. Most of the species occur in deep water, some in very deep water (_A. simplex_, 1600 to 1700 fathoms). In this and the following genera the coenenchym is {354}thin and the zooids imperfectly or not retractile. _Ceratoisis_, Wright; Atlantic Ocean, extending from shallow to deep water. The branches arise from the nodes. _Chelidonisis_, Studer; deep water off the Azores. _Isidella_, Gray; Mediterranean Sea. _Bathygorgia_, Wright; off Yokohama, 2300 fathoms. This genus is unbranched, with very long internodes and short nodes. The zooids are arranged on one side only of the stem.