Key To Genera

la. Pseudonodulus present (sometimes revealed only with EM) 2

lb. Pseudonodulus absent, central labiate process present (sometimes revealed only with EM) Azpeitia, p. 123

2a. Valves circular or elliptical 3

2b. Valves semicircular Hemidiscus, p. 128

3a. Areolation radial, usually fasciculate Actinocyclus, p. 117

3b. Areolation basically linear in central portion of the valve, fasciculate or radial in marginal part Roperia, p. 130

Genus Actinocyclus Ehrenberg 1837 Lectotype: Actinocyclus octonarius Ehrenberg (vide Boyer, 1927, p. 80).

VanLandingham (1967) listed 80-90 validly described Actinocyclus species of which only 4 or 5 are commonly recorded from marine or brackish water plankton. This chapter includes these 5 taxa and other marine planktonic species newly described or transferred to Actinocyclus.

Generic characters:

Valvocopula wide, a narrow second band with ligula, and a probable narrow third band.

Valve mantle deep and straight.

Valve outline circular.

Areola pattern usually fasciculate.

Marginal zone often denser in areolation and/or different in direction of the striae.

Central annulus, variable in size and presence. No labiate process in valve center. Marginal ring of labiate processes. Pseudonodulus, variable in size.

Characters showing differences between species: Type of fasciculation.

Position of labiate processes: at the end of edge and/or central areola row of a fascicle. Distinct or indistinct annulus. Distinct or indistinct pseudonodulus. Position of pseudonodulus.

The 11 Actinocyclus species dealt with here exhibit three different types of fasciculation:

A. Radial areola rows parallel to central row: A. actinochilus, A. circelltts, A. kiitzingii, A. ttormanii, A. octonarius, A. sagittulus, and A. subtilis (Table 24).

B. Radial areola rows parallel to edge (side) row: A. curvatulus and A. exiguus (Table 25).

C. Areola rows parallel to central and/or edge row: A. spiritus and A. vestigulus (Table 26).

TABLE 24 Morphometric Data of Actinocyclus spp. with Areola Rows Parallel to the Central Row of the Fascicles


Pervalvar axis (|Mn)

Diameter ((im)

Valve areolae in 10 |jun

Marginal striae in 10 |xm

H.m Between marginal proc.

A. actinochilus

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