It has been quite well determined, I think, that the typical forms of A. heliopelta occur at the base of the Miocene. At Rock Hall, Md., on the eastern shore of Chesapeake Bay, at a depth of from 21 to 130 ft., and at Wildwood, N. J., at a depth of from 78 to 179 ft., diatomaceous beds occur considered by Mr. Lewis Woolman (Geol. Surv. of N. J., 1898, pp. 116-121) "as synchronous in age," the former being deposited in the Delaware River Delta and the latter in the Chesapeake in post-miocene times. In each of these beds a small form of A. heliopelta is rarely found. The material at Buckshutem is post-miocene, and the form here figured shows a marked variation from the Miocene species and a gradual approach toward A. undulatus.
Valve circular, usually divided into fourteen sectors which are on the same plane at the centre, but the alternate ones are elevated into mammillated projections terminated by small processes on the margin. Zone view rectangular with undulations subconical, terminated by the processes.
Central space hyaline, rounded or slightly stellate, from which radiate rows of fine puncta in quincunx, shown in the figure only on the alternate elevations, the depressed interspaces being out of focus. The mammillæ are stated by Bailey to vary from six to ten.
Very rare in the blue clay (Walnut St. Bridge). Occurs also in the Wildwood deposit (Bull. Torrey Bot. Club, 1895, p. 261).
_Aulacodiscinæ._--Valves with mammiform elevations near the border surmounted by nipple-like processes.
(1) _Actinocyclus._--Valve with one small ocellus; striæ radial.
(2) _Eupodiscus._--Valve with one or more ocelli; striæ not radial.
(3) _Auliscus._--Valve with large, elevated ocelli. Central area hyaline. Markings granular and costate.
(4) _Pseudauliscus._--Valve with radiating granules. No central space.
{ 26}AULACODISCUS EHR. (1844) em. RATTR. (1888)
Valve usually circular, plane or with an elevated zone, frequently inflated beneath the processes; central space irregular or rounded, sometimes absent; markings granular, radial, sometimes in a reticulum.
The genus comprises more than one hundred species most of which are fossil, and is represented in this locality by a single form, _A. argus_, included by Rattray in his section "Retiformes," distinguished by the presence of a reticulum.