256 Mr. W. Clark on some undescribed Animals 



culum at some distance from the end of the foot, but the two 

 first turns of the spire are nearly obsolete ; the third occupies the 

 greater portion of the plate, and is well marked with oblique 

 lines of growth. 



I have lately examined many lively specimens, and can con- 

 firm the fact of the operculigerous lobe terminating in three 

 filaments, as well as the presence of the short mantellar process 

 that is produced and retracted, at the will of the animal, from 

 the upper angle of the aperture. What are the functions of this 

 organ is doubtful ; it has not the aspect nor is in the position of 

 a reproductive element ; it has more the resemblance of a ten- 

 tacular instrument ; but in some Rissoce it acquires an imperfect 

 tubular appearance, as in the Chemnitzia, in which, particularly 

 Ch. acuta, it seems to perform the office of the branchial siphon 

 of the Canaliferce. I believe that this appendage has scarcely 

 been noticed by authors; it appears to exist in many of the 

 Rissoce, but if in all is doubtful ; it has no connexion with the 

 operculigerous lobe, or its wings or caudal cirrhi, but is a strictly 

 mantellar process. The animal is free, unusually rapid on the 

 march, inhabits all the zones, and has not before been observed. 



Rissoa costata, Montagu. 



Animal inhabiting an elaborately sculptured, costated, spi- 

 rally striated, basally ridged pale yellow shell of 5-6 rounded 

 volutions, hyaline white, except the large black eyes and pale 

 red buccal disk. Head a long proboscidiform muzzle finely cor- 

 rugated in quietude, cloven vertically at the orifice as in R. parva, 

 but showing more partially than in that species the corneous 

 jaws and buccal apparatus. The mantle is plain and even. The 

 tentacula are long, flat, not filiform, rather thick at the base, 

 tapering gradually to a rounded extremity ; they are not setose : 

 the large eyes are fixed on prominences at the external angles. 

 The foot at rest is short, on the march it extends to the middle 

 of the antepenultimate volution ; it is labiated in front, but not 

 auricled, constricted above instead of in the middle, as is more 

 usual in Rissoa, and then expands and tapers to a narrowish 

 attenuated rounded termination. The operculigerous lobe di- 

 lates into subcircular lateral alse, bearing close at the junction of 

 the foot with the body, a suboval corneous faintly spiral oper- 

 culum with the turns rapidly increasing, as in the paucispiral 

 Littorina and typical Rissoa. It has a distinct caudal cirrhus. 



Malacologists, from the curious sculpture and entire flat stri- 

 ated broad margin of the peristome of the shell, have thought 

 that this hitherto unrecorded animal would display singular 

 features ; that is not the case ; it is a very simple creature, and 



