Mastogloia floridensis

Description – LM morphology:

In girdle view, frustules rectangular with rounded corners at valve apices and isomorphic rectangular partecta of valvocopulae extending to near apices (partecta density = 11–14 in 10 μm, n =21). Valves broadly lanceolate (smaller cells) to rhombic (larger cells) with cuneate sub-rostrate apices (Figures 2B, 7A, C, E). Valve measurements: length 49–86 μm, width 14–17 μm, length to width ratio 3.4–5.5, n =21. Axial area narrow, gradually expanding and becoming asymmetric around undifferentiated central area (Figure 2B). Small central nodule evident as bright area when focusing on valve interior (Figures 7B, D, E). External raphe branches sinuous (Figures 7A, C, F). External central raphe ends unilaterally curved and eccentric around central area (Figures 8A, C). Central raphe ends very closely spaced (Figures 2B, 7A, C). Valve pattern forming 2 zones within each hemivalve, a paraxial zone of parallel (valve middle) to convergent (most of length) alveoli and a lateral zone of radiate alveoli that extends onto valve margin, 15–17 in 10 μm. Longitudinal rows of alveoli separated by unornamented areas (Figures 2B, 7A, C, E). Partecta transapically elongated, rectangular (width = 1.8–2.5 μm) with very slightly convex inner margins, 11–14 in 10 μm (Figures 7B, C, F); terminal partecta distinct from rest, slightly longer with rounded inner margins.

Description – SEM morphology:

Valve faces slightly undulate with depression at valve middle and slightly elevated on each side of middle depression (Figures 8A, B). External valve face with single sinuous row of small, ovoid to dash-like pores on inner margins of paraxial alveoli, following contours of raphe branches (Figures 8A, C). Single linear row of small dash-like external alveolar openings on outer margins of marginal alveoli on mantle (Figures 8B, D black arrow). Paraxial and marginal alveoli separated by unornamented areas (Figure 8C). Moderately deep mantle gradually sloped at depressed valve middle and more steeply sloped towards apices without a distinct transition between valve face and mantle (Figure 8B). Sinuous raphe branches within narrow axial area, becoming unilaterally arcuate and slightly elevated from valve face in central area (Figures 8A, C). External central raphe ends terminating very close to each other (Figure 8C). External apical raphe ends simple, deflected towards the same side of valve as central raphe ends, extending onto valve mantle and terminating at margin (Figures 8B, D).

Internally, 1 to 2 rows of irregularly arranged punctate pores present on outer margins of each paraxial alveolus and on inner margins of each marginal alveolus (Figures 9A, C, and D). Areas between the internal alveoli pores unornamented (Figures 9C, D). A longer, apically elongated dash-like pore present just below valve margin at terminus of each marginal alveolus (Figure 9D arrow). Internal raphe branches straight (Figures 9A, C). Internal central raphe ends straight, simple, closely spaced (Figure 9C). Internal apical raphe ends simple, straight, surrounded by small helictoglossae (Figures 9B, E). Cingulum consisting of closed partectate valvocopulae and an additional unornamented open copula (Figures 8B, 9F). Each partectum perforated by six transapical linear rows of very small poroids (Figures 9B, F). Valvocopula with shallow septa at apices connecting with partectal ring (Figures 9B, E). Externally, partecta exit pores comma-shaped (Figures 9B, F), inner margin convex (Figure 9A).

Diagnosis – Valves with two sets of alveoli on the valve face, those near axial area parallel becoming convergent, opening externally by single pores adjacent to the raphe, separated from outer alveoli by unperforated lateral areas. Outer alveoli open externally by single pores on the mantle. Distinguished from Mastogloia frickei in having only one longitudinal row of external alveoli openings on each hemivalve face, alveolus openings on the internal valve face in the form of pore fields, and transapically-elongated partecta versus two longitudinal rows of external alveoli openings on each hemivalve face, alveolus openings on the internal valve face in the form of single pores, and nearly square-partecta for Mastogloia frickei.

• Author
  Lobban and Frankovich 2023
• Length Range
  49-86 µm
• Width
  14-17 µm
• Striae in 10µm
  15-17