PSEUDAULISCUS SPINOSUS (CHRISTIAN) RATTR.
Valve subcircular or slightly quadrangular, depressed at the centre and rising to an elevated zone near the border, the two zones separated by a distinct line. The inner zone indistinctly reticulate with fine puncta radiating from the centre and apiculi at intervals. The outer zone with smaller apiculi surrounding the inner zone and with intermingled rows of fine puncta and interrupted diverging striæ. Near each end of the minor axis is a rather long, robust spine inserted at one-fourth the radius from the border which is narrow and striated. Processes circular, close to the circumference.
The genus is named by Schmidt, described by Leuduger-Fortmorel and emendated by Rattray.
(_a_) _Triceratiinæ._--Frustule cylindrical or prismatic, with three or more sides.
(_b_) _Biddulphiinæ._--Frustule cylindroid; valve with ends elevated into round processes or long horns.
(_c_) _Anauleæ._--Valve elliptical, lunate or triangular, with internal septa.
(_d_) _Euodieæ._--Frustule cuneate in zone view; valve lunate.
(1) _Ditylum._--Frustule imperfectly siliceous. Zone with numerous divisions. Valve with central spine.
(2) _Trinacria._--Processes with sharp spines.
(dis, two, and tyle, a swelling, referring to the outline of the frustule)
Frustule quadrangular, convex at the ends. Valve triangular, with undulating sides, the angles ending in a sharp point surmounted by a bristle. Surface of valve convex at centre from which projects a long stout spine.
Valve with the angles separated from the central part by lines imitating septa. Surface with radiating lines of fine puncta.
Detached valves only have been found in the blue clay. The form is regarded as but slightly siliceous and, therefore, the zone or girdle not being found in the fossil deposits, I am unable to illustrate it from material in the vicinity. On Plate 38, Figs. 6 and 7, I have sketched the zone and valve views of specimens found recently at Vera Cruz and labelled by H. L. Smith Triceratum intricatum West. I can find no difference between the recent and fossil forms of the valves. The zone is covered with fine puncta in quincunx, not visible under ordinary illumination.
The form as figured in Plate 6 corresponds to the figure of Lithodesmium undulatum Ehr. in Van Heurck, and West, in describing the Triceratium undulatum Wm. Sm. (figured as T. striolatum), thought that his T. intricatum was distinct from Ehrenberg's form on the ground that the latter came from the "Bermuda" (Nottingham) earth and must be strongly siliceous. Lithodesmium is characterized by the envelopment of the frustules by a cellular membrane which does not appear, evidently, in Ditylum. D. brightwellii is distinguished by its crown of spines on the margin; otherwise it closely resembles D. intricatum.