Amygdale

Anhedral

Anhydrite

Anisotropic

Apatite

Aphanitic (or aphyric)

Argillaceous

Rocks comprising, or containing a significant proportion of, clay are described as argillaceous.

Belemnite

Birefringence

Light passing through a mineral is resolved into two permitted vibration directions. In an anisotropic mineral light travels at different velocities along the two vibration directions. The ray travelling in the slow vibration direction will be retarded compared with the ray travelling in the fast vibration direction. On leaving the mineral grain the two rays will interfere, producing interference colours.

When the slow ray exits the mineral grain the fast ray will already have exited the grain and travelled an additional distance know as the optical path difference, or retardation (Δ). This distance is proportional to the thickness of the mineral grain (t) and the difference in the refractive indices of the two vibration directions (n₁-n₂):

Δ = t(n₁-n₂)

The birefringence is the difference in the refractive indices of the two vibration directions (n₁-n₂).

When rotated under cross-polarised light anisotropic minerals display changing interference colours called birefringencecolours. The birefringence (n₁-n₂) of the mineral can be determined from these colours using a Michel-Levy chart.

Bivalve

Taxonomy

Phylum: Mollusca

Class: Bivalvia

Diagnostic features

Palial line/palial sinus

Adductor muscle scars

Growth lines

Dorsoventral symmetry (some exceptions, for example Gryphaea)

Two hinged valves

Umbo

Hinge

Gills (rarely preserved)

Stratigraphic range

Cambrian to present

Way of life

Bivalves have occupied many environmental niches, living in a variety of ways:

Epifaunal, infaunal, nektonic

Marine, freshwater, brackish

Filter feeders through gills

Majority of bivalves begin life in a planktonic larval stage

By looking at the shell shape and palial sinus of fossil bivalves it is possible to say something about its mode of life.

Advanced notes

Apparent increase in bivalve diversity over time (or is this just a preservational bias?)

Bivalves makes their shells out of calite, aragonites, or both

The adductor muscles are used to keep the shell of the bivalve closed. This means that when they are relaxed, the ligament between the valves pulls them apart, so the 'relaxed state' for bivalves is 'open'.

The depth at which infaunal bivalves burrowed can be inferred from the palial sinus. The more prominent the palial sinus, the deeper the bivalve burrowed. This is because the size of the palial sinus is indicative of the size of siphon needed, which in turn depends on the depth of the burrow (deeper burrow, larger siphon).

Specimens

Browse bivalves

Links

youtube: scallops swimming

Cambridge University Museum of Zoology: Lifestyles of bivalves

Yale Peabody Museum of Natural History: differences between bivalves and brachiopods

Learn about types of preservation

Also

Go to the front of the 1A Lab. A display on the right hand side (by the window) shows a number of bivalves in life position. Have a look at how they have adapted their shape and other features to their way of life.