l'lx- earlicr writers usually included the species now allotted to the family Melitodida 

 in the old -'•mus /sis, thus involving the classification of the Isida in considerable confusion 



K with lus usual clear insight, separated the " Me/itkaacea" and Isidinae 



lisünct subfamilies >>t' his Gorgonidae, using the characters of the axis cylinder. 



G tablished the families Isida, Mopsead : inellada and Keratoisida, all 



of which are n<>\\ included in the single family Isida. 



gav( .1 careful diagnosis "t' the family Isida, and divided it into the 

 Primnoisidina and Isidina. Later (1889), in the Challenger Report, 

 Wright and Studer defined the family as follows: 



"Colony consisting of a simple or branched axis. The axis consists ol calcareous and 

 horny (internodal and nodal regions; the branches when present arising from either the nodal 

 or internodal regions, sometimes anastomosing ; the axis solid or hollow, smooth, lluted or 

 inulate. Ih< ■ base of the axis calcareous and attached". 



ro avoid any confusion of certain species of the Melitodida with the Isida, this definition, 

 although substantially correct, may be modified for the purposes of the present work as follows: 



Gorgonacea with an axis composed of alternating calcareous and horny segments, the 

 former being amorphous and not composed of a mass of agglutinated spicules, and the latter 

 entirely horny without the admixture of definite calcareous spicules. 



S veral authors have proposed breaking up this family into subfamilies; hut with the 

 increase of our knowledge of the group this becomes increasingly difficult. Gray (1870) went 

 to the extreme of placing the species now included in the Isida into four distinct families, 

 viz. Mopseida, Acanellada, Keratoisida and Isida. 



VERRIL] 1883 separates all but the genus his into a family Ceratoisida, which therefore 

 includes Gray's Keratoisida, Acanellada and Mopseida. Studer (1887) divides the family into 

 the subfamilies Ceratoisidina \ Primnoisidina (including Mopseida) and Isidina. In their Report 

 on the Alcyonaria of the Challenger Expedition (1889) Wright and Studer substitute the name 

 Mopseina for Studer's subfamily Primnoisidina, and base the subfamilies of the Isida on the 

 characters of the spicules. 



The trouble with this arrangement is the intergrading features between the Mopseina 

 and Ceratoisidina. In his discussion of the genera Ceratoisis and Primnoisis, which belong to 

 the Ceratoisidina and Mopseina respectively, Hickson (1907) holds that these genera are nol 

 distinct, and proposes including both in the genus Ceratoisis. He says : 



- rhe study of many specimens belonging to this family has convinced me that this 

 subdivision is unnecessary and inconvenient. The many variations "I spicule characters that are 

 found in thi if a single genus render these structures unsatisfactory for the purpose ol 



systematic differentiation. If we take a single species from each of two of the subfamilies 

 and compare them, the differences observed in the character and arrangement ot the spicules 

 may >eem to be of a higher rank than the usual differences between genera; while, on the 



hand the examination of a large number of species of the same two genera will reveal 



of the intermediate conditions as to render the separation of the genera, on spicule 



lone, impossible. This kind of difficulty is particularly well seen in the ras.- ol the 



