. Bulletin. Science; Natural history; Natural history. 148 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. Figs. 2-3. Micrarionta rufocincta. 2, Shells (SBMNH 33896), basal and apertural views, diameter mm; varix at position of light streak to left of umbilicus. 3, Part of varix extending across base of body whorl (CAS 051774), x 10. lamellar layer with first-order lamellae perpendicular to growth lines and parallel to the direction of helical growth; (2) a middle, concentric crossed-lamellar layer with first-order lamellae oriented at right angles to the first and perpendicular to the direction
. Bulletin. Science; Natural history; Natural history. 148 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. Figs. 2-3. Micrarionta rufocincta. 2, Shells (SBMNH 33896), basal and apertural views, diameter mm; varix at position of light streak to left of umbilicus. 3, Part of varix extending across base of body whorl (CAS 051774), x 10. lamellar layer with first-order lamellae perpendicular to growth lines and parallel to the direction of helical growth; (2) a middle, concentric crossed-lamellar layer with first-order lamellae oriented at right angles to the first and perpendicular to the direction of helical growth; and (3) an inner, radial crossed-lamellar layer with first-order lamellae again perpendicular to the growth lines (Figs. 4-6). Second- order lamellae of the radial crossed-lamellar layers dip at angles of 28°-38° to the inner and outer surfaces of the shell. The contact between the outer and middle layers is gradational, with the first-order lamellae undergoing a 90° rotation in the plane of the shell surface; the contact between the middle and inner layers is sharp and unconformable. In areas of normal, completed growth, the inner and middle layers each make up about 40% of the thickness of the shell; the outer layer makes up about 20%. In general, where the outer two layers are thin, as at the beginning of a new growth increment, the inner layer is thicker so that the final shell thickness remains relatively constant. In juvenile shells the inner layer does not reach the growing margin of the aperture but thins to a feather edge within 1-2 mm behind it. The unconformity at the end of one helical growth increment and the beginning of another (Fig. 4, directly below the strong growth line, G) affects the outer and middle layers only. The inner layer passes under it without a break. Most of the thickness of a varix results from an increase in the depth of the middle, concentric crossed-lamellar layer. Several separate episodes of shell de- position may go
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