14 THE VOYAGE OF H.M.S. CHALLENGER. 



greatest importance, do not induce any perceivable changes in the conformation or appear- 

 ance of the body, as is easily seen by comparing with one another Elpidia glacialis 

 and Scotoplanes glohosa. Even if I had been able to study the madreporic canal in all 

 the forms of this family, I should certainly have hesitated to choose as generic characters 

 anatomical peculiarities which in most cases can be distinguished only with the greatest 

 difficulty and by means of particular microscopical research, excepting, of course, where 

 these peculiarities are accompanied by others. 



The variability in the position of the pedicels, which are sometimes arranged all along 

 each side of the body, sometimes wanting in the anterior half or third of the ventral 

 surface, and sometimes present only round the posterior extremity of the body, does 

 not give suitable generic characters. I have only once made use of this peculiarity of the 

 pedicels, when establishing the genus Scotoanassa, which is most peculiar on account 

 of its pedicels running out from the margin of the considerable brim which surrounds 

 the posterior extremity of the body. 



Thus, as stated above, there are great difficulties at present in producing a natural 

 classification of the family Elpidiidas. 



Tabular View of the Genera of the Family Elpidiid^;. 



I. Tentacles ten. 



A. Dorsal surface with processes most frequently of considerable size. 



a. Body very long, cylindrical, Synapta-shaped. Processes of the 



dorsal surface almost inconspicuous. Calcareous deposits 

 four-armed, with a single long central process, directed 

 outwards, . . ..... Gen. I. Parelpidia, n. gen. 



b. Body more or less elongated ovate, cylindrical, or depressed 



posteriorly. Processes of the dorsal surface generally large. 



1. Calcareous deposits : four-armed bodies with large pro- 



cesses, and, besides, sometimes small wheels, . . Gen. II. Elpidia, Theel. 



2. Calcareous deposits : unbranched spicula or three- 



armed bodies ; and, besides, always small C-shaped 



spicula, ...... Gen. III. Scotoplcmes, n. gen. 



3. Calcareous deposits : simple, irregularly curved, horse- 



shoe-shaped bodies, scattered straighter spicula, and 

 sometimes a few perforated, neUike plates — 



x . Madreporic canal opening externally, . Gen. TV. Kolga, Dan. & Kor. 



x x . Madreporic canal not opening externally, . Gen. V. Irpa, Dan. & Kor. 



B. Dorsal surface with a large lobe-like appendage anteriorly, and com- 



monly with some minute processes, ..... Gen. VI. Peniagone, n. gen. 



C. Body very depressed, almost flat, with a large brim round its anterior 



and posterior ends, ....... Gen. VII. Scotoanassa, n. gen. 



II. Tentacles eleven to twelve, ....... Gen. VIII. Ach!yonice,n. gen. 



III. Tentacles twenty, ........ Gen. IX. Enypniastes, n. gen. 



