I. Subfamily Conchasmida. Lateral edges of the two valves smooth, without teeth. |  | Valves without sagittal keel, nearly hemispherical or slightly compressed. |  | Aboral hinge without horns, | 720. Concharium. |
| Aboral hinge with two horns (one on each valve), | 721. Conchasma. | ||||
II. Subfamily Conchopsida. Lateral edges of the two valves dentate, with a series of prominent teeth on both sides. The teeth of both valves catch one into another. |  | Valves without sagittal keel, nearly hemispherical or slightly compressed. |  | Aboral hinge without horns, | 722. Conchellium. |
| Aboral hinge with two horns. No apical horn, | 723. Conchidium. | ||||
| Aboral hinge with two horns. Apex also with a horn, | 724. Conchonia. | ||||
| Valves with a sharp sagittal keel, strongly compressed on both sides, boat-shaped. | | Aboral hinge without horns, | 725. Conchopsis. | ||
| Aboral hinge with two horns (one on each valve). | 726. Conchoceras. |
I. Subfamily Conchasmida. Lateral edges of the two valves smooth, without teeth. | ||||||
| Valves without sagittal keel, nearly hemispherical or slightly compressed. | ||||||
| Aboral hinge without horns, | ||||||
| 720. Concharium. | ||||||
| Aboral hinge with two horns (one on each valve), | ||||||
| 721. Conchasma. | ||||||
II. Subfamily Conchopsida. Lateral edges of the two valves dentate, with a series of prominent teeth on both sides. The teeth of both valves catch one into another. | ||||||
| Valves without sagittal keel, nearly hemispherical or slightly compressed. | ||||||
| Aboral hinge without horns, | ||||||
| 722. Conchellium. | ||||||
| Aboral hinge with two horns. No apical horn, | ||||||
| 723. Conchidium. | ||||||
| Aboral hinge with two horns. Apex also with a horn, | ||||||
| 724. Conchonia. | ||||||
| Valves with a sharp sagittal keel, strongly compressed on both sides, boat-shaped. | ||||||
| Aboral hinge without horns, | ||||||
| 725. Conchopsis. | ||||||
| Aboral hinge with two horns (one on each valve). | ||||||
| 726. Conchoceras. | ||||||
Subfamily 1. Conchasmida, Haeckel.
Definition.—Concharida with the lateral margins of the two valves smooth, without interlocking teeth.
Genus 720. Concharium,[[336]] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.
Definition.—Concharida with the lateral margins of the valves smooth, without sagittal keel and without horns on the hinge.
The genus Concharium is the simplest and the most primitive form of all Concharida; it may be regarded as the common ancestral form of the whole family. The entire shell is usually almost spherical, without horns or teeth, and may be regarded as a Castanella which is bisected or broken into two equal hemispherical halves. The lateral margins of the two hemispherical valves are smooth, without teeth, and catch one into the other like the two valves of a Diatom, or the two halves of a bivalved box. Concharium agrees in this simple shape of the frontal margins with the following genus Conchasma, and represents with it the small subfamily Conchasmida.
1. Concharium bivalvum, n. sp. (Pl. [123], figs. 2, 2a).
Shell spherical, smooth. Diameter in all directions nearly the same. Borders of the two hemispherical valves circular, smooth, about twice as broad as the pores. In the half frontal perimeter of the shell (along the right and the left border of each valve) twenty-two to twenty-four pores, in the half sagittal perimeter (in the middle line of each valve) eighteen to twenty-two pores, in the half equator (in the cinctural perimeter of each valve) twenty to twenty-two pores. All pores circular, of the same size, twice as broad as their bars.
Dimensions.—Length of the shell (longitudinal diameter) 0.35, height (sagittal diameter) 0.34, breadth (lateral diameter) 0.33.