412 Mr. W. Clark on British Mollusca. 



margin. I have them all mounted. Again, they are all strictly lit- 

 toral species, though by mistake they have been stated to have oc- 

 curred at sea. These are striking resemblances as to the organization 

 of the soft ])arts ; but, strange to say, they all differ in the form of 

 the shell, one being cylindrically elongated, two globosely conic, and 

 the fourth discoid. 



This is another instance, amongst others, in which the form of the 

 shells does not assist us in discriminating their respective species. 

 Though I object generally to the change of old generic and specific 

 names, I think that the present generic title of Truncatella points 

 too exclusively to one of its species, and it might be advantageously 

 changed : this is an exce[)tional case : I would therefore propose that 

 this very interesting group, in which all the animals, though the 

 shells diflFer, are identical in every essential character, should bear 

 the generic appellation of Assiminiu (Gray, Leach) ; but if Dr. Gray 

 should think it inexpedient to allow of any enlargement of the generic 

 characters oi Assiminia to receive Philippi's three species (viz. T. 

 littorea, 2\ truncatula, and T. atornus), I defer to such high autho- 

 rity. And then Philippi's genus Truncatella must receive his three 

 original species, unless only the conic T. littorea be transferred to 

 Assiminia ; in that case the T. truncatula would stand as the type 

 of the genus : still we must have a new one for T. atomus (certainly 

 not Euotnphalus). Even such an arrangement will have its incon- 

 veniences, from the animals being identical, though inhabiting dif- 

 ferently formed shells. This last observation is Philippi's, who ob- 

 serves, in his 'Moll. Sicil.' vol. i. p. 13.3, "This animal inhabits 

 different shells, viz. those that are subcyliudrically decollated, globoso- 

 conic, and even discoidal." 



Mr. Jeffreys appears to have drawn largely on his imagination in 

 considering this minute species the last living representative of the 

 fossil Euomj)hali, and in comparing it with E. pentanffulatus. The 

 characters of our little shell do not agree with the fossils, especially 

 with respect to the aperture ; but any of the Planorbes, or even our 

 discoid Rissoa {Shenea Flanorhis), have more pretension, if any at all 

 exist, to be called the living representative of the Euomphali than 

 our little Truncatella atomus : both the above-named are flat on one 

 surface and umbilicated on the other ; and in many of the Planorbes 

 the aperture is subangular, as in Euoinjihalus, but in the T. atomus 

 it is suborbicular. I hope this summer to see the animal alive, and 

 I have no doubt that it will bear me out in the observations I have 

 offered * . 



* 1 should be greatly obliged by any gentleman sending me, by the post, 

 some of these minute animals, as soon as captured, to Bath, before the 

 20th of May, 1 859, and after that date, to Exmouth, Devon ; they should 

 be put in an ounce-and-a-balf bottle, quite full of fresh clear sea-water, 

 enclosed in a tin cylinder ; for the cost of which and postage, I would 

 forward a Post-Office order. I have omitted to state that these minute 

 creatures, not exceeding, with their shells, ^^^th of an inch in diameter, 

 are a littoral species, and feed on the Codium tomentoaum or other minute 

 Alga3 in the rock-pools. 



To find them, put the Alga; in a sieve, in a pan of sea-water, with 



