Ceratoisidinae: - - Ccenenchyma thin; calyces inserted, long, usually cylindrical; characteristic 

 spicules in the form of spindles, or flattened bar-like of hour-glass forms which are smooth 

 or with minute spiny points, never with true verrucae; colony not profusely branched. 



M o p s e i n se : - - Ccenenchyma thin ; calyces usually exserted ; often club-shaped ; characteristic 

 spicules flattened scales, often profusely branched or ctenate on their edges. 



Systematic relationships of the family ISID^E. 



The closest affinities of this family seem to be with the Primnoida, a fact which has been 

 commented upon by several writers, the calcareous internodes of the former being apparently 

 very similar in composition and structure to the calcareous axis of the latter. The ccenenchyma 

 is usually thin in both families. 



In the polyps and calyces of certain Isida we have a very striking similarity to those of 

 the Primnoidcz, and this is particularly true in the case of the new genus Peltastisis described 

 in this Report. Here the form of the calyx, the form and arrangement of the spicules, the 

 reduction in the number of adaxial scales, and the essential character of the operculum (a scale 

 to each segment) are all typically primnoid in their nature. Were it not for the jointed axis 

 of Peltastisis I doubt if any one would hesitate in putting it in the family Primnoidcz. 



Indeed the character of the axis alone can be successfully used in separating these two 

 closely related families. 



On the other hand, the Isida show many characters in common with the sclerogorgian 

 family Melitodidee. These families can not be separated on the characters of the spicules, for 

 those of the Isinfs show the same general characters as are common in the Melitodida. The 

 nodes and internodes are superficially much alike in the two families, and the calcareous inter- 

 nodes are in some cases, particularly in the older parts of the colony, apparently almost as 

 completely calcareous and amorphous in the Melitodida as in the Isidte. It was formerly held 

 that both nodes and internodes of Melitodidtz were penetrated by solenia ; but this is now 

 denied a . Here, again, the characters of the axis seem to afford the only reliable means of 

 family differentiation, the horny nodes of the Melitodida being always more or less mixed with 

 calcareous spicules, while those of the Isida are purely horny, without spicules. 



The writer desires to emphasize an opinion elsewhere expressed 2 as follows : 



"Our knowledge of the lower invertebrates is still far too fragmentary to permit us to 

 indulge the hope that our work as systematists will really serve to express in final form the 

 actual relationships of any large group of animals. 



While a natural classification is a 'consummation devoutly to be desired' we are not yet, 

 it seems to me, in sight of it". 



The main service of classification, in our present state of knowledge, is to facilitate 

 identification of groups, the final result being facility in determination of species. 



1 BOURNE. A Treatise on Zoology, Part II, Chapter VI, p. 28, 1900. 



2 The Gorgonacea of the Siboga Expedition, III, The Muriceida;, 1900, p. 9. 



