Chama (The Jewel Box Clam)

The bivalve (clam) genus Chama Linnaeus, 1758 belongs to family Chamidae Lamarck, 1809. The geologic record of the genus is Paleocene to Recent. Extant (living) species are found mostly in shallow-tropical waters (intertidal to 100 m depth). There are a few species, however, that live in cool waters (e.g., in Oregon and California). The common name of this bivalve is the “Jewel Box.” 

Chama is a small to medium-size heterodont bivalve, which means it has only a few teeth, and they are subdued and somewhat degenerate. This is because Chama normally cements itself to the substrate (e.g., in crevices within coral-reef rock), therefore, large teeth are not needed.

Chama lazarus Linnaeus, 1758 is a moderately common species of Chama found in water depths ranging from 1 to 20 m, in the Indo-Pacific region. The specimen shown below is from the Philipine Islands.

This first image shows both valves closed together (articulated). The shell is 75 mm height (including projections of the spines) and 55 mm width (including projections of the spines). The right valve is on the left side of this image. During life, the right valve would be facing upward, so as to discourage a predator from biting down on the sharp spines. 

This second image shows the exterior of the right valve, whose sharp bladed spines are encrusted by some patches of calcareous red algae, small worm tubes, and bryozoan colonies (nearly microscopic).

This third image shows the exterior of the left valve, which is attached to a piece of colonial coral. The left valve, which is the "lower" valve, also has some bladed spines, but they are absent where the valve makes contact with the substrate. Thus, this bivalve always has its sharp spines present on any vulnerable part of its shell.

This last image shows the interiors of both of the valves (left valve on the left and right valve on the right). The brown material just above the teeth is the thin ligament, which helps hold the valves together.

A Dolomite Ventifact

A ventifact is a stone that has been shaped by the erosive action of windblown sand (rarely ice crystals), bouncing along (saltating) only a few feet above the ground. The abrading grains of sand “file” (pit, etch, striate, groove, rill, and/or polish) away at the rock surface, which acts as a windbreak, and as result a sharp edge (facet) forms. Ventifacts form in arid environments (deserts) where there is a strong wind and a steady (but not overpowering) supply of sand or ice crystals. Windblow processes are referred also to as aeolian processes.  

Ventifact rock type (lithology) is commonly a very hard material like quartzite, chert, obsidian, or some basalts. Less common is dolomite, whose chemical formula is CaMg(C0₃)_2. Dolomite reacts only weakly with cold, dilute hydrochloric acid (HCl). Limestone (CaCO₃), which is another carbonate rock, is much softer than dolomite and, unlike dolomite, reacts vigorously with cold, dilute hydrochloric acid.

The ventifact shown below is a stone of dolomite (22 cm width and weighing about 2.5 pounds). Where it was found is unknown, but it was probably found in a desert in southern California.

The first view shown below is the top view of the stone. Most of the grooves are on the left side, but, if you look closely, you can see a smaller set of grooves on the lower right-hand area. Although this smaller set is in parallel alignment with the other set, the smaller set is at a sharp angle relative to the larger set. The only way to explain this phenomenon is that there had to been a change, over time, of 180° in wind direction. The facet (a sharp edge of demarcation between the two different-angled sides of the upper surface of the rock) is readily visible. 

For your information, the planet Mars has been modified by aeolian processes more than another other solid planet in our Solar System. One half of the rocks found at the Mars Path Finder Vallis Landing site in 1996 are ventifacts, and they were formed by northeast to southwest winds. For more information about that site, see the following article (free for reading):

Bridges, N.T., et al. 1999. Ventifacts at the Pathfinder landing site. Journal of Geophysical Research v. 104, no. E4, pp. 8595–8615.