Middle Ordovician – Early Devonian
Trochonema is an extinct genus of facultatively mobile epifaunal suspension feeders
Identification in Hand Sample:
- Base generally flattened yet sometimes quite inflated
- Shell shaped like a spinning top or inverted cone
- Growth lines crossing the whorls from above obliquely backward
- Presence of lower spiral angulation
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- Turbiniform to aciculate, with a major spiral angulation having channel within; later whorls disjunct in some forms.
Ulrich & Scofield (1897):
- Shell turbinate, umbilicated; spire varying in height, base generally flattened yet sometimes quite ventricose; whorls not numerous (4-8), varying from strongly angular to rounded, always with two more or less prominent ridges or angles between which lies a broad vertical, usually flat or concave, peripheral space; a third ridge usually near the suture, while a fourth generally surrounds the umbilical cavity. Other, but smaller ridges may occur though chiefly on the basal half of the whorls. Lines of growth crossing the whorls from above obliquely backward, often vertical and not infrequently inclined in the opposite way on the peripheral band. In the last case the outer lip is broadly notched at the extremity of the upper peripheral angle. Aperture usually very oblique; peritreme complete; inner lip varying in thickness, not reflected. Type, T. umbilicatum Hall sp.
- This excellent genus exhibits considerable variety in its contents. There is scarcely a single character that pertains strictly to all the species, yet, comparing one with the other, we find them so closely knit together that to separate them very far would mean nothing less than violence to natural classification. We do not mean to say that the genus may not be conveniently and yet naturally divided, or subdivided, if that is preferable. Eunema, Salter, the type species of which is scarcely more than a high and practically imperforate Trochonema, was established at the same time as Trochonema. As at present understood, Eunema is not a well-marked genus, while its employment even as a subgenus is attended with difficulties. It cannot be maintained on Salter’s characterization, since in shells of this type the relative size of the umbilicus is a very unreliable generic character. However, if we will select another character of E. strigillatum, viz.: the very slight obliquity of its aperture, the group may have some value. A much better division may be instituted for the reception of the group of species typified by our T. pulchellum which eventually gave origin to Cyclonema. The separation of this group is certainly convenient since it leaves Trochonema (s. s.) as a more compact and sharply defined group.
Nettelroth (1889) :
- This genus is never defined by any writer in this country. It includes spiral shells, having some relations to Pleurotomaria, but differing from that genus by exterior characters.