« This species, exceedingly small, has the test tapered like a needle, slightly swollen at its aperture, which has a spike on one side and a triangular indentation on the other: we do not know if the opposite end is pierced or not. The two membranous wings of the animal are slightly waved on their edge, and of a soft pinkish colour. In the interior of the test we can see only pink and red filaments. » – Quoy & Gaimard: “Observations zoologiques faites à bord de l’Astrolabe…”, Annales des Sciences Naturelles vol. X, Paris 1827, p.233.

« What idea can one make of the myriads of individuals that this species must feed in the sea, when one sees, for example, that a net is never thrown in all the zone indicated without bringing back an infinite number? It is, without a doubt, as common to it alone as all the others together. It was not unusual to take a few hundred at a time in a net a foot and a half in diameter, less than a quarter of an hour of troll; but this species, however common, is not diurnal. Only a few isolated and strayed individuals are ever taken by day, and only when clouds hide the sun; but as soon as evening twilight begins, it is the first species which appears on the surface, and the moment when it is more numerous closely follows the beginning of twilight; for it diminishes in number as soon as the night is closed. One also takes some specimens in the morning. It swims briskly, holding its shell in an oblique position. It does not flutter with that vivacity that we have described in the H. quadrispinosa, and executes its motions on a more regular line of march. » – A. D. d’Orbigny: Voyage dans l’Amérique méridionale… Paris 1835-1847, vol. V p.120.

A specimen, wings retracted, from 1m deep, uvala Stari Trogir, Sevid area, Split-Dalmatia Comitat, W. Croatia. Notice the visceral mass, partially coloured in brownish red, and the dorsal groove that extends throughout the adult shell. For a more detailed overview of the anatomy, you can consult Souleyet in the Voyage de la Bonite, Zoologie part II, p.193 and plate VIII fig.5 of the Atlas.