Page 15 - Land Snails of New Mexico

|- parietal
upper palatal
lower palatal
angulo-parietal
palatals
basal
Figure2A,Shellsshowingsomegeneralfeatures:Left,therelativelyhighshellofRabdotusdurangoanus(15.8mm high)and,right,therelatively lower,widershellofAshmunellarhyssa(16.8mm wide). B,C,D,Ventralviewsofthreeshellsshowingrelativedevelopment,orabsenceof, umbilicus,B,umbilicate(Zonitoidesarboreus,5.4mm wide),C,perforate(Glyphyaliniaindentata,5.6mm wide),andD,imperforate(Vitrina pellucidaalaskana,5.9mm wide). E,ShellofPupillamuscorumshowingterminologyappliedtowallsoftheaperture.Thepalatalandbasalwalls comprisetheouterlipandthecolumellarandparietalwallstheinnerlip. F,G,H,Diagramsshowingterminologyappliedtoteeth(denticles)ofpupillid shells.F,Vertigoelatior(2.2mm high),G,Vertigoovata(2.3mm high);andH,Gastrocoptaprocera(2.2mm high).
narrow shells,asnotedabove,thecolumellatendstobemore like a real column, and the umbilicus is highly restricted or obliterated. It is obvious that there cannot be much of a
columella in such low, broad shells. It m a y exist as a short, diminuitive structure in the apical whorls, but as whorls soon widen, theircolumellar walls become separated from each other by increasing distances, so that in such a shell, the umbilicus will become much wider than inhigh narrow shells. This can be seen inviewingtheshellfrombelow.Theumbilicusmay appearas a deep, well-like hole in shells with narrower umbilici and grade from this morphology to a broad, circular depression in those with wider umbilici, as mentioned above for discoid shells.
Even inshellsthathave well-developed umbilici, thisfeature may be partially or entirely obscured from view by the overgrowth of shell material from the adjacent columellar wall. A shellinwhichthereisnosuchovergrowthisreferredtoas being umbilicate. Ifthe umbilicus is partially covered, perhaps leaving only a small "chink" open, the shell is referred to as being perforate. Ifthe columellar wall completely covers over the umbilicus from external view, w e have an imperforate shell
(Fig.2B-D).
An importantpartoftheshell,insofarasidentificationis concerned,istheapertureanditsrelatedstructures.The aperture is s o m e t i m e s c o m p a r e d to a m o u t h a n d its r i m to a circular lip, also called the peristome (Fig. 2E). The term peristome means literally"around the mouth," and is used to refer to the margin of the aperture, in general, but sometimes is used to imply a margin inwhichthickeninghastakenplace.Inmany kindsofshells,part or all of the peristome becomes progressively thickened upon reachingmaturity.Thisthickeningiscausedby thelayingdown of calcareous shell material, called callus. The wall may be referred to as being calloused. A terminology has been developed (Figs, 2E-G) for the aperture, which divides itinto four parts. Recalling that the aperture is like the lower opening of a cone, these four parts are, in general, like the top, bottom, and two sides of this opening. The top and inner sides are
sometimes termed the inner lip, and the outer and bottom parts, the outer lip. Both the inner and outer lips commonly are subdividedintotworegions.The lowerpartoftheinnerlipisthe columellar wall, already discussed. Obliquely or horizontally oriented, above the columellar wall, is the parietal wall. In apertureswiththinlips,theparietalwallmay benothingmore