Quoy and Gaimard, 1833
Trunk very elongate or flat (Fritillaria aequatorialis, Fritillaria aequatorialis 2). Oral part of the trunk covered by a membranous hood. Mouth with lobes. Stomach spherical. Pyloric part of the stomach often with glandular appendices, intestine and rectum below and behind, obliquely to the right, or extending to the right of the stomach. Gonads behind the digestive tract. Tail with a broad fin, strongly indented near its root.
Lohmann and Bückmann (1926) recognized subgenus Acrocercus (Fritillaria fraudax) and subgenus Eurycercus (Fritillaria borealis) mostly on the basis of the shape of the tail fin. However, given that this structure is extremely fragile in this group, and frequently damaged in net samples, it is preferable to begin the identification of the material by observing the characteristics of the trunk (Fenaux, 1967).