the example of Atlanta plana above, the adult shell can be seen to possess a total of 4-1/4 whorls. Although this total whorl count may be useful as a taxonomic character for describing fully-grown adults, it is variable in all other individuals since the number increases with shell growth. The number of whorls comprising the shell spire, however, is not variable, but rather is predictable because it represents only the larval portion of the adult shell. In late-stage larvae (e.g., see the A. plana larval shell above), the last 1/4 whorl increases noticeably in width by comparison with the preceding whorls. In post-metamorphic shells with raised sculpture on the spire, the junction between the larval shell (protoconch) and the beginning of the adult shell (teleoconch) is often marked by the termination of, or a change in, the sculpture pattern (e.g., see the SEM of the tilted shell spire on the A. brunnea page). Even without the ability to distinguish this point of transition (as is often revealed clearly in SEMs), the point at which the whorl width increases markedly is visible under the dissection microscope. The sketch below of the whorl sutures from an adult A. inflata is used here to describe the method for determining the number of whorls comprising the spire. Note that in practice, the shell is first oriented so that the central embryonic part is directed upward in the field of view. Then, the whorls are counted outward using an imaginary line directed upward (as shown below). In the case of A. inflata, the number of spire whorls at the point of rapid increase in whorl width is 4-1/2. Examples of whorl sketches for two other species, A. lesueurii and A. peronii (from Richter and Seapy, 1999), are shown below for comparison. Spire whorl numbers for the nineteen species of Atlanta (included in the table below) range from two and one-half (in A. lesueurii and A. oligogyra) to six (in A. gibbosa).
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Figure. Sketch of the whorl sutures from Atlanta inflata. The shell is oriented with the embryonic shell directed upward; the whorls are then counted based on an imaginary line directed upward from the center of the embryonic shell. The numbers in the sketch denote the end of one whorl and the beginning of the next one. The arrow indicates the number of whorls (4-1/2) comprising the spire. © 2009 Roger R. Seapy
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Click on an image to view larger version & data in a new windowFigure. Sketches of the whorl sutures on the shell spire and extending onto the first adult shell whorl in Atlanta lesueurii (left) and A. peronii (right). The dashed line in each sketch denotes the ending and beginning point for each spire whorl. The arrow at the bottom of each sketch indicates the number of whorls comprising the shell spire. © 2009
- The shape of the spire is highly variable among Atlanta species and can often serve as a distinctive character (see below). Examples include the tall, turreted spire of A. turriculata, the conical spire of A. brunnea, and the tilted (or "inclined") spires in the A. inclinata and A. gibbosa species groups.
- In addition to spire shape, spire ornamentation can also be very distinctive; ranging from smooth (e.g., the A. lesueurii species group and three of the members of the A. peronii species group) to ornamented by low, elevated spiral ridges (e.g., A. inflata, A. helicinoidea, and A. plana), to highly ornate and elevated sculpture (e.g., A. brunnea, A. echinogyra and A. turriculata).
- Nineteen species are currently regarded as valid in the genus Atlanta. These species are partitioned into seven species groups (see table below). Tesch (1908) was the first to group together the species of Atlanta sharing similar morphologies. He recognized four species groups; the A. peronii-, A. inflata-, A. turriculata-, and A. inclinata-groups. In addition to these, three additional ones are currently recognized; the A. lesueurii-, A. gaudichaudi- and A. gibbosa groups. Except for Tesch's (1908) A