204 RETUSA. 



Cylirhnimi MONTS., Norn. Gen. e. Spec., p. 143, 1884, type B. 

 umbilici il' i Mont. 



Shell small, sub cylindrical, imperforate, with slightly raised, flat, 

 or depressed spire, the aperture as long, or nearly as long, as the 

 shell, narrow above, dilated below. Columella thickened, with a, 

 small fold or none. 



Animal (pi. 60, figs. 1, 2, 3, R. truncatula ; pi- 60, fig. 5, R. niti- 

 dula) capable of being retracted into the entirely exposed shell, the 

 head-shield short, produced backward in two narrow lateral tentacu- 

 lar processes. Radula wanting. Gizzard provided with three small, 

 elliptical corneous plates, irregularly tuberculate-dentate on their 

 inner faces, the tubercles longer near one end of the plates (pi. 60, 

 fig. 7, stomach of R. nitidulus containing gizzard-plates and fora- 

 niinifera. Fig. 6, plates of the same species. Fig. 8, plates of R. 

 umbilicata. Fig. 4, one plate of R. truncatula). 



This genus differs from Cylichna in wanting radula-teeth, in the 

 posterior processes of the frontal disc, in the peculiar gizzard plates, 

 and in the exposed spire of the shell. 



The species of this genus might be distributed into two groups, as 

 Fischer has done. Part of them have a distinct fold on the colum- 

 elhi, as in Tornatina, and for these the genus Retusa Brown was 

 proposed, and also Coleophysis Fischer, the types of both being 

 />///// trum-iihila Brug. The other species have no distinct col urn- 

 ellar fold, although the pillar-lip is thickened ; and these fall into 

 I'h-irti/n.* as understood by Sars, Fischer, Dall and others. As this 

 name is preoccupied in zoology, it must be rejected ; and if the 

 division is to be retained a new name must be coined for the forms 

 like iilifu.i'1. jtrrtrnuis, etc. The value of the distinction seems to me 

 to be hardly worth a name, however, as the strength of the coliun- 

 ellar fold is subject to great mutation, and it would be very difficult 

 to decide upon the position of certain species in which the fold is 

 slight, thus bridging the gap between the extreme forms of either 

 ^ri]. Dall's proposition to make " Coleophysis " a subgenus of 

 'itinn and Utriculus a distinct genus with Retusa as a subgenus 

 cannot be adopted, being barred by taxonomic canons. Monte- 

 roaato'> u r roup (JijHchnhm has generally been placed in or next 

 to Cyliclnm, but, according to Sars's observations, belongs to Rttit*>i 

 It i- :!i-tin^ui*hed by the narrowly, deeply umbilicated aj-x. 

 What arrangements may be made when the soft parts'of these small 

 dwellers in the deeps come under scalpel and microscope, cannot 

 now be guessed ; the only thing certain is change. 



