RISSOA. 359 



with muddy yellow or orange above, but brighter on the 

 under surface. The tentacula are very long, white, and 

 slender, clothed with distinct horizontal setae from base to 

 point, which become more rare towards their termination. 

 Eyes black, on minute yellow prominences at the external 

 angles. Foot very long and narrow; the front portion is 

 greatly extended on the march, subrotund, slightly labiated 

 and scarcely aurieled, terminating posteally in a lanceolate 

 point; the centre of the sole has a depression, accompanied 

 by a posterior medial line ; on each side the upper surface, 

 about the middle, is a well-marked line of lead-colour. The 

 operculigerous lobe is very distinct, though not much alated 

 anteally, and terminates in an expansive rounded membrane, 

 which, like the foot, is marked on each side with a blotch of 

 dark lead-colour, but of greater width ; these run into and 

 almost unite on the upper surface, forming what some authors 

 term a posterior dark spot. At the anterior junction of the 

 foot with the body there is a transverse yellow or orange 

 band, which is seen through the sole ; the operculigerous lobe 

 emits a very long and distinct caudal cirrhus, as much so as 

 in R. parva, and it carries close to the posterior part of the 

 foot-pedicle, a suboval, white, obliquely striated, paucispiral, 

 corneous operculum. 



The animal in every respect represents a typical Rissoa ; it 

 is remarkably lively and free, and affords every facility for 

 examination. It marches with great rapidity, and inhabits 

 abundantly the coral zones of the South Devon coast at 

 Exmouth, in an offing of six miles. 



I have stated in the ' Annals of Natural History/ vol. vi. 

 p. 33. N. S., that I thought this species a coralline zone va- 

 riety of R. parva ; I withdraw that opinion, as I am satisfied 

 of its distinctness. 



The preceding observations were written in 1851, but in 

 the summer of 1852 I had the good fortune to meet with 

 some rare unrecorded animals of the British Rissoa. To 

 show that an account of these minute species is a desidera- 



