« Shell orange to brown, slender to inflated cylindrical, less glossy [than S. nicaeensis], smooth. Found on Alcyonium glomeratum. Mostly found in the Mediterranean, mainly from Italy and Croatia, at 50 to 200m, but also reported from Galicia, northwestern Spain, at 20m (Fehse et al. 2010). The exact distribution in the Atlantic is unknown. » – Lorenz & Melaun: “A new species of Simnia from England (Caenogastropoda: Ovulidae)”, Molluscan research vol. 31(3), 2011, p.170.
 
In fact, other species with different anatomy by with almost identical shells do exist in Eastern Atlantic, for example the Xandarovula jacintoi (Fehse & Trigo, 2015), with which shells of X. aperta can be confused.

« Testa laevigata elongato-ovali, fulva-rufescente; canalibus breviusculis; apertura aperta, labio externo tenui, antice sub-emarginata; labio interno postice ad canalem sub-reflexo, antice sub-tortuoso tenui; intus sub-depresso. » – G. B. Sowerby: Thesaurus conchyliorum vol. II - text, London 1855, p.478. Above, the species in the Thesaurus, plate CI.
 
« Of a form between that of O. triticeum, and that of secale; of a dull reddish-brown colour. In the specimen which we have, the lip is not thickened nor reflected, although it appears to be a full-grown shell; if it be so, it will form a companion to O. Patulum. » – Ibid.

Synonym purpurea Risso, who lists two differences with “nicaeensis”: a shorter and wider canal, a constantly purplish colour… – A. Risso: Histoire naturelle des principales productions de l’Europe méridionale… vol. IV, Paris 1826, p.235. As this name purpurea was published without any drawing or usable description, it is considered as a nomen dubium. Locard’s wrtitings about Ovula purpurea is a little more interesting: « Shape fusiform, slightly elongated, equally narrowed on top and bottom; last whorl very tapered at the extremities, well rounded in the middle; aperture rather narrow, broadened at the base; labrum simple, barely thickened inside; columellar margin with an oblique fold at the top; channel short and wide; columella thickened, slightly arched; colour of a light red, peristome paler. » – A. Locard: “Les coquillles marines des côtes de France”, Annales de la Société Linnéenne de Lyon n.s. t.37, Lyon 1891, p.36.

« It differs from the previous species [the “nicaeensis” of Risso]: by its smaller size; by its globally more swollen curve; by its shorter and more dilated canal; by his test striped only at the extremities; by its purple colouration, etc. » – Locard & Caziot: “Les coquilles marines des côtes de Corse”, Annales de la Société Linnéenne de Lyon n.s. t.46, Lyon 1899, p.219.

Jeffreys (British Conchology vol.5 p.222) considers Ovula purpurea as a junior synonym of Ovula patula Pennant. Monterosato (Notizie intorno alle conchiglie mediterranee p.54) considers also purpurea as a variant of patula. But purpurea has a constant purplish colour and a narrower shape.