Acestrorhynchus microlepis

Actinopterygii: The Actinopterygii, or ray-finned fishes, constitute a class or subclass of the bony fishes. The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii. In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish. (Wikipedia)

Freshwater: Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, icebergs, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and other total dissolved solids. The term specifically excludes seawater and brackish water. (Wikipedia)

Demersal: The demersal zone is the part of the sea or ocean (or deep lake) comprising the water column that is near to (and is significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of the larger profundal zone. (Wikipedia)

Tropical: The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at 23° 26' 16" (or 23.4378°) N and the Tropic of Capricorn in the southern hemisphere at 23° 26' 16" (or 23.4378°) S; these latitudes correspond to the axial tilt of the Earth. (Wikipedia)

Curved: The overall shape of the ichthyolith curves in one direction.

Cavity: The ichthyolith has a central nerve cavity. This is a base keyword included whenever an ichthyolith has a cavity, even when the cavity is further categorized with an additional keyword.

Triangular: The base of the ichthyolith is the widest part.