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Xenascus blastema
Xenascus blastema, Davey, 1970, p. 356; emendations Stover and Helby, 1987
Originally Odontochitina, subsequently (and now) Xenascus.
Taxonomic senior synonym: Hystrichosphaera (as Phoberocysta, now Xenascus) ceratioides, according to Davey and Verdier (1971, p.27) — however, Stover and Helby (1987a, p.128) retained Xenascus blastema.
Holotype: Davey, 1970, pl.5, fig.4
Locus typicus: Minieres Borehole, Escalles, Pas de Calais, France
Stratum typicum: Cenomanian
Original description: Davey, 1970 (as Odontochitina blastema):
Diagnosis:
Central body ovoidal; outer membrane forming long tapering apical horn and two shorter tapering antapical horns. Periphragm smooth or slightly punctate; in region of central body gives rise to small number of branched precingular and postcingular processes. Apical region commonly lost in archaeopyle formation.
Description:
O. blastema is of the typical Odontochitina shape but possesses a number of subsidiary processes. These processes vary considerably in size and shape. They may be finely acuminate or more usually they branch, either distally or proximally, and terminate with a small bifurcation. When well developed they are seen to be in two series encircling the central body. One series (precingular) lies just posterior to the archeopyle margin, while the other series (postcingular) is situated towards the posterior end of the central body. Between the two series, corresponding approximately to the widest part of the central body there is an area devoid of processes marking the position of the cingulum. In one specimen (pl. 5, fig. 5) two processes are present also on the proximal part of the larger antipical horn.
Dimensions:
Holotype: width of central body 48 µm, length of antapical horns 54 and 61 µm, length of subsidiary processes 8-20 µm.
Range: length of apical horn 78 µm (one specimen); length of antapical horns 50 (55.7) 61 µm, width of central body 41 (44.4) 51 µm, length of subsidiary processes 4-23 µm.
Number of specimens measured = 5.
Affinities:
The presence of well developed subsidiary processes easily differentiates O. blastema from all previous described species in this genus. Since the overall form of the shell is identical to that of other members of Odontochitina, it was considered proper that this species should be placed in this genus. Hystrichosphaera ceratioides Deflandre is similar in size, overall form and possesses precingular and postcingular processes. However, this species possesses a distinct tabulation.
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Originally Odontochitina, subsequently (and now) Xenascus.
Taxonomic senior synonym: Hystrichosphaera (as Phoberocysta, now Xenascus) ceratioides, according to Davey and Verdier (1971, p.27) — however, Stover and Helby (1987a, p.128) retained Xenascus blastema.
Holotype: Davey, 1970, pl.5, fig.4
Locus typicus: Minieres Borehole, Escalles, Pas de Calais, France
Stratum typicum: Cenomanian
Original description: Davey, 1970 (as Odontochitina blastema):
Diagnosis:
Central body ovoidal; outer membrane forming long tapering apical horn and two shorter tapering antapical horns. Periphragm smooth or slightly punctate; in region of central body gives rise to small number of branched precingular and postcingular processes. Apical region commonly lost in archaeopyle formation.
Description:
O. blastema is of the typical Odontochitina shape but possesses a number of subsidiary processes. These processes vary considerably in size and shape. They may be finely acuminate or more usually they branch, either distally or proximally, and terminate with a small bifurcation. When well developed they are seen to be in two series encircling the central body. One series (precingular) lies just posterior to the archeopyle margin, while the other series (postcingular) is situated towards the posterior end of the central body. Between the two series, corresponding approximately to the widest part of the central body there is an area devoid of processes marking the position of the cingulum. In one specimen (pl. 5, fig. 5) two processes are present also on the proximal part of the larger antipical horn.
Dimensions:
Holotype: width of central body 48 µm, length of antapical horns 54 and 61 µm, length of subsidiary processes 8-20 µm.
Range: length of apical horn 78 µm (one specimen); length of antapical horns 50 (55.7) 61 µm, width of central body 41 (44.4) 51 µm, length of subsidiary processes 4-23 µm.
Number of specimens measured = 5.
Affinities:
The presence of well developed subsidiary processes easily differentiates O. blastema from all previous described species in this genus. Since the overall form of the shell is identical to that of other members of Odontochitina, it was considered proper that this species should be placed in this genus. Hystrichosphaera ceratioides Deflandre is similar in size, overall form and possesses precingular and postcingular processes. However, this species possesses a distinct tabulation.
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