Diagnosis.– Primitive Hendersonia, with incipient test wall flexure on the lateral sides of the earlier portion of the test.

Description.– Test biserial throughout. Proloculus is small (8.8-11.0 µm), followed by 12 to 15 chambers that in- crease gradually, slowly to moderately in size as added. Chambers are globular in lateral view, overlapping at var- ious rates; those of the earlier portion of the test are sub-rectangular in shape (Pl. 1, Figs 1, 6, 9), excepting the last formed one or two, petaloid. Chambers are subglobular to globular in edge view (Pl. 1, Figs 4, 7, 8), gradually increasing in size as added. Sutures are distinct and de-pressed, straight and slightly oblique to the test axis of growth. Periphery is subacute in the earlier portion of the test due to the development of a test wall flexure (Pl. 1, Figs 1, 4, 8, 9); in the absence of the test wall flexure, last- formed chambers present rounded periphery. Aperture is in the shape of a medium-high arch situated at the base of the last formed chamber. Two small, imperforate to microperforate, symmetrical flanges border the aperture; flanges can be simple or rimmed (Pl. 1, Figs 3, 6). Chamber surface is ornamented with costae with variable width (2.3-3.1 µm). The earlier part of the test can be smooth; a periapertural pustulose area is consistently present (Pl. 1, Figs 1, 3, 4, 6, 7-9). Test wall calcitic, hyaline and microp- erforate to finely perforate; pore size varies between 0.7 and 2.4 µm (Pl. 1, Figs 2, 5, 6, 9).

Remarks.– Hendersonia hendersoni nov. sp. consists of two test varieties. The “primitive”  variety (Pl. 1, Figs 1-5, 8-10) is dominant at lower latitudes (e.g., South Atlantic Ocean) and presents resemblances with the ancestral species of the Heterohelix planata (Cushman, 1938) group in general test appearance, finer costae (3.3 to 3.7 µm) and small pores (0.7-0.9, rarely up to 1,1 µm). The “evolved” variety (Pl. 1, Figs 6-7) is dominant in the tropical waters (e.g., southern USA, Yucatan outer shelf). It presents thicker costae (3.6-4.1 µm) and larger pores (1.0-2.4 µm), resem- bling by these features the descendant species, Hendersonia carinata (Cushman, 1938). The two test varieties record the incipient stage of evolution of the Hendersonia and the initiation of the lineage is apparent in the increase in costae width, pore diameter and consistent occurrence of incipient test wall flexure on the lateral sides of the early part of the test, the latter giving the appearance of keeled periph- ery. Hendersonia hendersoni differs from  Heterohelix planata in having coarser costae, larger pores and incipiently developed test wall flexure on the lateral sides of the earlier parts of the tests. Cushman (1938, p. 10, pl. 2, figs 4-5) described Gümbelina globocarinata from the Campanian of Texas (USA). Examination  of the type specimens of this species (NMNH, Cushman Collection, Washington, D.C.) showed that according to the test periphery, which is not carinate, and the general test architecture, chamber shape and aperture shape and periapertural structures, it should be included within the Heterohelix globulosa (Ehrenberg, 1840) group of species (Fig. 5).

Phylogenetic relationships.– Hendersonia hendersoni evolved from a species of the Heterohelix planata group; it is the ancestor of Hendersonia carinata (Cushman,1938) (Fig. 4).

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