Neidium fossum

Neidium fossum K.Lefebvre & P.B.Hamilton, sp. nov. (Figs 18, 20, 31–34, 43–47)

Individuals examined for morphological analyses: 27, examined for molecular analyses: 1. Valves are elliptic-lanceolate, with narrowing to subacute apices (Figs 31–34). Valve length 92–142 μm and width 22.5–34.0 μm. Central area is transapically broad and elliptical, covering 1/3 to 1/2 the valve width. The axial area from centre to apex is linear-elliptical. In LM, the raphe appears filiform and linear-elliptical, in SEM the raphe is linear. Proximal ends form hooks in opposite directions and the distal raphe ends form a lacinia (Figs 43–47). Striae are parallel and moderately spaced becoming weakly convergent towards the apices, 17–21 in 10 μm. Voigt faults are present on the secondary side of the valve. Areolae are regularly spaced and irregular in shape, 17–20 in 10 μm. In SEM, externally the areolae are flat to the valve surface sometimes with cribra occlusions and not in deep depressions (Figs 43–44). Internally, the areolae are positioned within small depressions (Fig. 47). One large longitudinal canal is present along each valve margin (Figs 43–47). Additional canals 2–3 or more may extend along the valve mantle. Externally the longitudinal canal is flat with the valve face (Fig. 43), internally the canal forms a bulge along the valve margin (Fig. 46). A single row of linear to linear-elliptical areolae open externally and internally from the canal (Figs 45–46). Internally, areolae are covered by a hymen (Fig. 45). Renilimbia are positioned around areolae along the axial area, and the region of the longitudinal canals (Figs 45–46). Renilimbia can also be found randomly scattered around areolae on the valve face. Areolae form chambers in the valve wall that are highly interconnected transapically (Fig. 47, Supplemental Plate 2, Fig. f). Each proximal helictoglossa forms a double elevated ribbed nodule that is loosely connected to the adjacent helictoglossa by a thin ridge of silica (Figs 45, 47). The terminal helictoglossae form on small thickened nodules close to the apex (Figs 46, 47).

Etymology:—The specific epithet (fossum), Latin meaning ditch, trench or canal, refers to the large longitudinal canal seen in N. fossum

Observations:—Neidium fossum was identified from four of Ehrenberg’s samples, (Andower MA, Bridgewater MA, Pelham MA and Stratford CT), where it had been listed under either N. dilatatum or N. amphigomphus (Figs 18, 20, Supplemental plate 2). From the Ehrenberg samples, the size ranges of N. fossum observed were length: 90–158 μm, width: 22–40 μm and 19–26 striae in 10 μm. Within the modern samples observed in the phylogenetic study, N. fossum (Figs 31–34) has a similar overall shape to N. dilatatum (Figs 23–24), however N. fossum is much smaller than N. dilatatum with a maximum length of 160 μm. This species can be differentiated from N. amphigomphus, by its narrowly rounded ends, and longitudinal canal structure of one large and several small canals (Figs 43–47). It can be differentiated from N. dilatatum (Figs 35–38) with its large longitudinal canal, smaller overall size, more transapically elliptical central area and its straight bifurcating terminal raphe ends (Figs 43, 46). There is taxonomic confusion around large linear-elliptical Neidium taxa with multiple longitudinal canals extending along the valve margins. One clade which is represented by one larger canal with associated smaller canals, can be linked to N. fossum. Specimens identified as Neidium iridis sensu auct nonull.—Camburn & Charles (2000: 21, fig. 2), Metzeltin & Lange-Bertalot (2009: 93, figs 1–4, 94, figs 1–9)—and as Neidium amphigomphus—Metzetlin & Lange-Bertalot (2002: 51, fig. 1), Metzeltin & Lange-Bertalot (2007: 183, fig. 5, 188, fig. 3, 189, fig. 2), Metzeltin & Lange-Bertalot (2009: 95, figs. 1–3)—are N. fossum or closely related undescribed cryptic species. Other similar specimens identified under N. iridis include Patrick & Reimer (1966: 34, fig. 1) and Krammer & Lange-Bertalot (1986: 104, fig. 2).

• Author
  Lefebvre and P.Ham. 2015
• Length Range
  92-142 µm
• Width
  22.5-34.0 µm
• Striae in 10µm
  17-20