This paper is the first in a monographic study of the cephalopod family Cranchiidae. Thirteen of the 41 nominal genera are recognized as valid for the family: Cranchia, Liocranchia, Leachia, Helicocranchia, Bathothauma, Sandalops, Liguriella, Taonius, Galiteuthis, Mesonychoteuthis,
Egea, Megalocranchia and Teuthowenia. A full definition is given for the family and each valid genus; diagnoses are given for the two subfamilies Cranchiinae and Taoniinae. A generic key is accompanied by illustrations of a representative species of each genus. A comprehensive historical
resume for the family is given and generic interrelationships are discussed. The subfamily Taoniinae comprises roughly three groups of genera based primarily on the development and relationships of the posterior end of the gladius (lanceola and conus) and the fins and on the photophore
pattern on the eye. Helicocranchia, Bathothauma, Sandalops and Liguriella form a loose group of the more primitive genera that possess a short conus (lost in Bathothauma) and small subterminal or terminal fins. Taonius, Galiteuthis and Mesonychoteuthis form
a group of specialized genera in which the conus and associated terminal fins are elongate and the large suckers of the club are modified into hooks or suckers with one or two large, central, hook-like teeth. Of these, Galiteuthis and Mesonychoteuthis are closely related. In
the third group formed of Egea, Megalocranchia and Teuthowenia, the fins elongate and simultaneously extend up the lateral margins of the mantle as the conus elongates. Egea and Megalocranchia are closely allied. The great variety of sexual dimorphism of the
arms in the family is compared. All species of the relatively homogeneous Cranchiinae possess a hectocotylus in the male and brachial end-organs in females. In the more diverse Taoniinae, the males lack a distinct hectocotylus but have a high incidence of secondary sexual modifications of
some arms and the females of only one group, Egea-Megalocranchia-Teuthowenia, possess brachial end-organs.
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