The valves are also in some species very unequal in their respective thickness, as may be seen in Productus (Daviesiella) 1 llangollensis, Davidsonia verneuilii, &c., and while the space allotted to the animal is very great in many species, as in Terebratula sphaeroidalis, it is very small in others belonging to Stro phomena, Leptaena, Chonetes, &c. The ventral valve is usually the thickest, and in some forms is six or seven times as great as the opposite one.
Traces of the original colour have also been preserved in some of the fossil forms; radiating bands of a reddish tint have been often seen in wellpreserved examples of Terebratula (Dielasma) hastata, T.
The ventral valve is usually the larger, and in many genera, such as Terebratula and Rhynchonella, has a prominent beak or umbo, with a circular or otherwise shaped foramen at or near its extremity, partly bounded by one or two plates, termed a deltidium.
Terebratula (Liothyris) vitrea.
In the Testicardines, of which the genus Terebratula may be taken as an example, five or six pairs of muscles are stated by A.