UMBONELLA VERRUCOSA. 319 



by concentric striae. Specimens in this condition are 

 extremely beautiful. With age the walls become dense 

 and opaque white. The umbo is a very conspicuous 

 feature; it is blunt and smooth, and very prominent. 

 The avicularium is often almost hidden behind it; in some 

 cases it is wanting, and I have met with a large colony 

 on which scarcely one was to be found. While the avicu- 

 larium is, in some cases, half-concealed and difficult to 

 recognize, in others it stands out prominently and pro- 

 jects to some extent over the aperture. 



When developed in deep water, U. verrucosa takes on a 

 very different character. The walls are dense and solid, 

 the ridges, which are continuations of the septa between 

 the areolar spaces, become much thicker, and often rise 

 into nodules and spinous projections ; and the umbo is 

 massive and roughened. The strongly marked ridges 

 radiate from the margin to the umbo; and the space 

 between them is occupied by a rather deep, loop-shaped 

 depression. The ovicell, which I have never observed on 

 the littoral variety, partakes of the same character : it is 

 thickly perforated in front ; and the punctured space is 

 surrounded by a calcareous border, which rises into a 

 number of spinous processes, forming a very handsome 

 crest (Plate XXXIX. fig. 2). 



Deep-water specimens are highly calcified, and the 

 appearance of the species undergoes a corresponding 

 change, but the leading features are unaffected. 



The orifice, which does not change with age, is arched 

 above, and the lower margin is almost straight or slightly 

 sinuous. On each side of it the front wall is carried up 

 for some distance, terminating above in a blunt extremity. 



HABITAT. On the surface of rocks, stones, shells, 

 Laminaria-Toots, &c. between tide-marks, and in shallow 

 water, and on shells &c. from deeper water (40 fathoms). 



