POLYPS AND CORALS. 59 



tentacles. From the former it differs in its elevated 

 column, regular form and large size of the disk, elon- 

 gated tentacles, and the absence of conspicuous cinclidse 

 near the base. 



It is the only genus of Actinidce, except Adamsia (^A. 

 palUata), in which a solid secretion is formed by the basal 

 disk. In Gancrisocia it has a concentrically striate struc- 

 ture, the striee being evidently lines of, growth. The 

 mode of formation seems to be this. The crab when very 

 young selects a very small fragment of shell or pebble, 

 which it holds upon its back by its posterior claws, as 

 other species of crabs {Hypoconclm) do a valve of Pec- 

 ten or some other bivalve shell. Upon this small shelly 

 or stony fragment the very young Oancrisocia finds a 

 congenial abode, but soon growing too large for its 

 station it enlarges its support by depositing a layer of 

 horn-like material, secreted by the base, around its cir- 

 cumference, and this process is continually repeated in 

 proportion to its own growth and that of the crab that 

 carries it. In this way there is soon formed a broad thin 

 pellicle, having its concentric elements arranged around a 

 nucleus of stone or shell, which is usually excentric, the 

 increase having been more rapid in front than behind. 

 This basal secretion is held upon the back of Dorippe by 

 its recurved posterior legs, in the same manner as the 

 original bit of shell. 



Gancrisocia expansa Stimpson, 1. c, p. 376, 1855. 

 Plate 2, figures 1, la. 



Base very broad, oblong oval, the sides somewhat 

 curved and the ends sub-truncate and rounded ; margin 

 creuulated. Basal secretion tough, brownish, strongly 

 striated concentrically, the nucleus usually excentric. 



Column smooth, in expansion elevated, diminishing 

 rapidly from the base to the middle, above which it ex- 

 pands again to the disk, which is smaller than the base ; 

 the diameter of the middle of the column equalling half 



