80 J. D. MACDONALD ON THE CLASSIFICATION OF THE GASTEROPODA. 
type of a separate family, with some more truly marine examples of which I am well 
acquainted. Many Truncatellce live far inland, exhibiting a terrestrial habit as obviously 
as any of the operculate Pulmonifera ; and their anatomical characters are well worthy of 
a rigid comparison with those of Acicula. 
The Patelllda? exhibit many points of affinity to Fissurella, Nerita, and the series to 
which they belong. Several members of the family manifest a tendency to deviate from 
the typical species, in their dental formula in particular ; and indeed the whole lingual 
armature in Patella appears but to represent the rachidian area of Turbo or Nerita, — the 
pleural teeth being suppressed, as in the mooted cases of JEJlysia, Eolis, &c, noticed in 
the first part of the paper. 
The affinity here advocated is still further suggested by the peculiar sculpturing, 
olivaceous colouring, and nacreous lining of the shell, in some species of Acmaa. 
The Patellida? and Dentaliidce have a broad upper-lip mandible, and their ear-sacs con- 
tain otoconia, but the resemblance ceases here ; while on the other hand in the Chitonida 
no upper-lip mandible exists, and I have never been able to detect either visual or audi- 
tory organs. I must give up the attempt to class the two latter families ; but I think that 
the Patellidce may be placed provisionally at least as an appendix to the Fissurellidce. 

I have no desire to repeat former observations, and therefore refrain from any further 
comment on the particular families contained in the Table. I can only say that I have 
endeavoured as much as possible to make the order of their arrangement in strict accord- 
ance with their anatomical relations. With all its imperfections, therefore, the schemt 
is submitted to the judgment of the zoologist, purporting to be merely a help to the 
attainment of a precise conception of the natural affinities of Gasteropoda. 
