APPENDIX. 519 



mediate space. The aperture is suborbicular, not thickened 

 at the outer margin as in the Rissoce, and almost entire. 

 The umbilicus is conspicuous. Axis -^ T) diameter -^ uncise. 



Animal. Ground colour of the palest muddy yellow. 

 Mantle even with the shell. Muzzle short, smooth, thick, 

 though flat above, cloven at the end, mouth beneath, with a 

 vertical fissure. Tentacula finely setose, divergent, flat, rather 

 slender, gradually tapering from base to point, which is 

 rovuided, hyaline-white, suffused with minute snowy points ; 

 eyes large, but rarely exserted, placed on low prominences at 

 the external angles of the tentacula. The foot has three 

 phases ; when at rest it is rounded anteally, and still more so 

 posteallyj when the animal swims it becomes narrow and 

 rounded to a similar extent and form at both ends ; on the 

 march it is arched convexly in front, not auricled, and pos- 

 teally has a blunt lancet-shaped termination; the sole is 

 deeply impressed with a longitudinal central line ; the oper- 

 cular lobe is short, but laterally alated, and carries, near the 

 extremity, a grossly spiral, yellow, rissoidean, subcircular, 

 corneous operculurn, which has been described as not spiral. 

 This is an error. In clean examples under the microscope, 

 the depressed nucleus and usual spiral turn may be traced ; 

 a dozen examples have been examined, and though obsoletely 

 impressed, they are clearly paucispiral. There is 110 distinct 

 cirrhus springing from the operculigerous lobe, but in some 

 specimens I have fancied I saw a very short blunt one. 

 Sometimes the point of the main sole is bifid for a very short 

 distance. The animal is most abundant on the littoral levels 

 at Exmouth ; it marches with great rapidity, and is vivacious 

 and free ; on the march the eyes are always under the shell, 

 as are usually the muzzle and foot, the ends of the tentacula 

 only being visible; it carries its shell at an angle of 70. 

 I have elsewhere stated that this species can scarcely remain 

 in the genus Rissoa, but these various rectifications induce me 

 to withdraw that opinion ; it really appears to have as many or 

 more rissoidean attributes than some of the species admitted 

 into that genus. 



