72 
VARIATION IN SCULPTURE. 
though ill many other species the spiral sculpture or striation is 
existent, but not often distinctly perceptible to the naked eye. 
The spiral, like the transverse, striation is soinetiines incised and not 
elevated, a character which is naturally not so noticeable and promi- 
nent as the elevated sculpture, but is clearly and distinctly visible 
with a lens upon the siirfiice of many species, oflering in Ifel/\v pisana 
one of the many characters distinguishing it from Jleliv virgata. 
In the Pelecypods the greater or lesser intensity and development 
of the concentric strim, which are the representatives of the trans- 
verse lines of accretion in the Univalves, are, broadly speaking, the 
only moditication the sculpture undergoes. 
The youthful stages of shell growth have also their special 
characteristics, both in Gastropods and Bivalves; the nuclear or 
apical whorls in Gastropods always 
increasing more rapidly in size and 
being often sculptured or adorned 
in a different manner to the later 
growth, have been mistaken for 
and even described as different 
species from the parent. Jleliv 
(tcniedta has spirally sculptured 
embryonal whorls, Pupti cplindracea an irregularly reticulate pattern 
thereon, and other species possess more or less distinctive and special 
characters. The umbones of some Bivalves also exhibit analogous 
peculiarities, those of Vmo having a complex arrangement 
of nodules and nodular ridges 
(see p. 42, tig. 102), those of 
rulo pieturum two rows of 
radiately divergent tubercles 
on each valve (see page Gd, 
tig. IdG), and A]U)dii)it((, a 
concentric series of ])romi- 
nent angulated ridges and 
obscure nodulations ; these 
peculiarities being owing to 
the permanent shell de- 
veloping within the primitive 
or glochidian shell of the embryo, the teeth of which prevent the 
symmetrical and regular growth which characterizes the species 
Fig. IGIK— /V/rr cylindracea Da Costa X ‘MK 
Isle of I.isniore, Main Argyll, 
Collected by iVIr. K. Staiulen, 
Showing the tlellcaie reticulate sculpture 
distinguishing the embryonal shell. 
Fit.. Anodonia anaiina var. coiuplanata 
Uossm. X 8, 
(lunifrieslon, near Tenby, 
Collected by Mr. F. Walker, 
Showing the umbonal sculpture characteristic of 
the species. 
