426 Dr. J. E. Gray on the yeniis Assiminia. 



groups. All the several Ti'uncatellcB found in general collections j 



of shells have these characters ; and I have seen the animals of ■ 



four or five species of the genus Assiminia from different parts I 



of the world, and there are in collections several species of shells, " 



which from their characters are believed to belong to the same 

 group. 



I am amused at Mr. Clark's quoting an opinion expressed by 

 ray excellent friend Dr. Philippi in 1841, as a reply to my ob- 

 servation in 1855. I feel assured that if he were in this country, 

 he would be the first to repudiate such a use being made of his 

 name. He is undoubtedly the most enlightened of the con- 

 tinental conchologists. Since that period he has much extended 

 his knowledge of the animals of shells, and is now personally 

 engaged in examining the species found on the coast of Peru; 

 and in his most recent work he regards Assiminia and Trunca- 

 tella as distinct genera belonging to different families ! (See 

 Handbuch, 173, 263.) Thus his latest work contradicts Mr. 

 Clark's views instead of supporting them. 



At the same time I may observe, that the mere external forms 

 of the animals of several genera are exceedingly similar; and 

 unless they are very particularly examined by a person well 

 versed with their peculiarities, what prove to be important cha- 

 racters may easily be overlooked, especially if the animals are of 

 a small size, such as can only be seen by the aid of a microscope. 

 For this reason I am not inclined to place much reliance on Dr. 

 Philippi's determination as to the animal of the species which 

 he has referred to the genus, when he informs us that the animals 

 are all hyaline, that the shell of one of them, Truncatella litto- 

 rina, is three-quarters of a line in length, and the discoidal 

 shell of the other, T. atomus, is scarcely a quarter of a line in 

 diameter, and that he was obliged to magnify it sixty diameters 

 to observe it. 



I think 1 need only quote the various genera, to which those 

 who have examined the animal of the only British species of 

 Truncatella have referred it, viz. Cyclostoma, Paludina, and lastly 

 Rissoa (to which Dr. Philippi referred it even after he had 

 figured it), to show that naturalists who consider the animal as 

 the only basis of a generic character, are more unstable in their 

 opinions than those who regard the shell and operculum as of 

 similar value in a scientific point of view. I may add, to show 

 how comparatively imperfect was Dr. Philippi's idea of this 

 mollusk, in the account of the animal quoted by Mr. Clark as 

 published in 1841, and again in 1844, that he considers it 

 necessary to point out how it differed from the terrestrial genus 

 Cyclostoma, and the fluviatile Paludina, both belonging to very 

 different families according to his present views. 



