From Fensome et al., 2019:
Gonyaulacysta jurassica ssp. adecta var. longicornis (Deflandre, 1938, p.171, pl.6, fig.6) Downie and Sarjeant, 1965, p.115. Emendation: Sarjeant, 1982b, p.31, as Gonyaulacysta jurassica subsp. adecta var. longicornis. Holotype: Deflandre, 1938, pl.6, fig.6; Jan du Chêne et al., 1986a, pl.37, figs.7–8. Originally Gonyaulax jurassica var. longicornis (Appendix B), subsequently Gonyaulacysta jurassica var. longicornis, thirdly Gonyaulacysta jurassica subsp. longicornis, fourthly (and now) Gonyaulacysta jurassica subsp. adecta var. longicornis. Taxonomic junior synonym: Gonyaulax (as Gonyaulacysta) jurassica subsp. brevis, according to Sarjeant (1982b, p.31). Age: Oxfordian.

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Original description (Deflandre, 1938) (translated from French):

Gonyaulax jurassica Deflandre var. longicornis n. var.
Plate VI, Fig. 6.

This variety differs from the type only in its general shape, the tabulation remaining the same as far as I could ascertain. The epitheca is elongated conical and extends into a long horn, nearly twice as long as in the type. This elongation is mainly due to the development of the apical plates, but the pre-equatorial plates also appear to contribute to the more slender appearance of the epitheca. The sutured ribs, in the few individuals observed, and especially in the one drawn as the type, are less developed and less spiny, especially compared to the very rounded specimens of G. jurassica.
The type of the variety longicornis measures 88 μm long, 54 μm wide, and the horn alone reaches nearly 30 μm.