26 craniidjE. 



adapted to the angles and extent of the basal surface. 

 When it has bare standing-room only, it increases in 

 height and becomes regularly conical. The under valve 

 of specimens attached to the smooth shell of a Pinna is 

 usually a mere film. The brachial fringe can be pro- 

 truded slightly beyond the margin of the shell at each 

 side, but never in front or at the back. It may be 

 likened to the spokes of two wheels, each placed on its 

 nave within a circle ; and as the spokes are nearly equal 

 in length, it is evident that at the point where the wheels 

 approach each other, the inside spokes project into the 

 space between the wheels, and not outwardly. There 

 are no cirri at the back. The lower or flat valve con- 

 tains only the base of the adductor muscle, upon which 

 as a pivot; the upper valve turns by a semirotatory 

 but very confined motion. The arms and rest of the 

 body are enclosed in the upper or convex valve. The 

 animal is by no means timid. When a camePs-hair 

 brush is thrust between the gaping valves, they imme- 

 diately close, but in a few seconds after open again; 

 and this teasing experiment can be repeated many times, 

 without alarming the Crania, or making it sulky. The 

 cirri are not retractile, and do not withdraw or shrink 

 when touched. Each arm has about sixty of them. 

 The fry are quite white and semitransparent, and they 

 have only a few tubular perforations. They adhere in 

 the same way as their parents. Their appearance is not 

 unlike that of the very young of Anomia ephippium. The 

 largest specimen in my cabinet measures over four-fifths 

 of an inch in diameter. 



Montagu called this species Patella distorta ; and it 

 has borne many other names, both generic and specific, 

 the latest being that of Criopus Orcadensis, given to it 

 by Dr. Leach. 



