An ancient "sundial"

The common name of the gastropod genus Architectonica Röding, 1798 is the "sundial" snail. There are140 extant (recent) species, and they have distinctive discoidal shells with a high "spiral staircase" in the center of the underneath side of the shell. Architectonica is widespread today, and most of the species live between 40°N and S, in subtropical to tropical marine waters (e.g., Indo-Pacific, East Africa, Japan, Hawaii). Although they range in depths between intertidal and abyssal, they are most commonly found in 10-65 m depths. They are carnivorous gastropods and prey mainly on sea anemones, sea pens, and corals.

Architectonica is classified as belonging to the family Architectonicidae Gray, 1850, which, in turn, belongs to a poorly resolved group of snails known as the "lower heterobranchs," whose larval shells are diagnostic in their morphology.

The following three images are of a recent shell of Architectonica perspective (Linnaeus, 1758), from Oman. The views are dorsal, apertural, and ventral, in the order they are shown. The specimen is 48 mm in diameter and 25 mm in height.

The next two images are of the fossil Architectonica cognata Gabb, 1864, of middle Eocene age (approximately 48 million years old), from southern California. The views are dorsal and ventral, in the order they are shown. 

The central region of this specimen is "plugged up" with hard siltstone. Careful cleaning is needed in order to expose the delicate shell underneath. Cleaning of fossil specimens demands expertise, otherwise, critical features can be damaged.

The geologic range of Architectonica is Paleocene to Recent. Architectonicids do not occur in older rocks (Mesozoic and Paleozoic).

For those who seek more detailed information (i.e., I used this reference for many of the above facts), see:

Bieler, R., and R.E. Petit. 2005. Catalogue of Recent and fossil taxa of family Architectonicidae Gray, 1850 (Mollusca: Gastropoda). Zootaxa 1101:119 pp.