VARIATION IN SCULPTURE. 
71 
sideied a young stage of the typical form ; it is probably, however, 
like Helix fuldielhi, one of our few dimorphic species. In Hyalinia 
radlatula the sculpture is transversely incised, appearing to radiate 
from the apex to the periphery, like the spokes of a wheel. 
When this incremental sculpture is distinctly developed at some- 
what regular intervals into more or less distant costulate ridges, and 
the spiral sculpture is also elevated and widely separated, the in- 
tersection of the elevated spiral and transverse lines causes a sunken 
or flattened area to be perceptible, 
forming the sculpture known as 
decussate or malleate (so called on 
account of presenting the appearance 
of being caused by light blows of a 
hammer or other instrument). These 
malleations or facets are very often 
(piite regular in their arrangement 
and necessarily dependent on the 
direction of the elevated striation to which they are owing. The 
variety lacumsd of Lunmm palmtrh is a characteristic example. 
Macgillivray ascribes the rough 
and corrugately malleate sculpture 
to e.xposure to drought, stating that 
under such circumstances the shell 
becomes wrinkled and “ marked with 
long ridges or irregular sinkings like 
the skull of a New Hollander.” 
The shells of the Llnma uuv in- 
habiting tropical and sub-tropical 
countries are said to be usually 
much more constant and uniform in sculpture, as well as in size and 
shape, than their congeners from more 
northern districts, and their texture is 
also finer and smoother on the whole 
than that of species living in the 
colder regions. 
Spiral or revolving sculpture is rare 
in British shells, Cydustonm elecjam 
being almost the only species which 
clearly and strongly exhibits it, 
FiC. 159. —External surface of body 
whorl of Cyclostoiua elcgans Mull., 
showing the strong spiral ribs and less 
prominent transverse striation (highly 
magnified). 
Fic. 158 . — LiDDUPa pcrcgra Midi, x Iw, 
Maidenhead, Berks., 
Collected by Mr. C. G. Barrett, 
Showing the corrugately malleated 
surface. 
P’lG. 157. — Lrnnuea palusiris var. 
lacunosa Taylor, 
Stream, Leventhorpe Pastures, Leeds, 
Showing a regularly decussate or mal- 
leate surface. 
