It is composed of silica spicules, which provided structural support and deterred enemies. Hydnoceras specimens are typically around 8.375 inches long, and about 3.5 inches wide. Hydnoceras specimens fossilize as internal molds of the actual sponge's skeleton, and the rectangular latticelike ridges are an impression of the sponge's skeleton.
Reef builder. Hydnoceras species were suspension feeders, and likely remained attached to a single substrate for their entire lives, as typical for a sponge. Unlike modern glass sponges, which typically live in deep marine environments, these sponges lived throughout the marine water column.
Glass sponge (this name refers to all of class Hexactinellida). Class Hexactinellida is still extant but is now found only in the deep ocean. In the past, they could be found at almost all depths. Hydnoceras is a good index fossil and is usually preserved as internal molds.