Of the eight recognized genera of Recent stomatopods, Lysiosquillc@ is probably the most heterogeneous and most difficult to define satisfactorily. To this genus have been assigned those species that have the carapace devoid of carinae and without a complete cervical groove; the abdomen flattened dorsally and without longitudinal carinae on the first five somites; the telson without a distinct median carina; the antepenultimate segment of the raptorial claw grooved for its entire length and not produced proximally beyond its articulation with the preceding segment; the penultimate segment of the claw finely pectinate or spinose along the outer part of its dorsal edge; and the terminal segment armed with four or more teeth and not in flated proximally. The nearly 40 known species and subspecies of Lysiosquilla in clude the largest and some of the smallest stomatopods. Only seven of them have been previously recorded with certainty from the western Atlantic, and none have been added to the genus from this region since 1900. L. scolopendra (Latreille, 1825) must be treated as a doubtful species, possibly synonymous with L. excavatrix Brooks, 1886, and L. plutnata and L. tnaiaguesensis, both described from immature specimens by Bigelow in 1901, were correctly transferred to Pseudosquilla by Schmitt (1940). The larval forms collected by the “? Challenger― Expedition off St. Vincent, British West Indies, and variously called Coronis (Erichthus) @ninutus, Lysiosquila (Coronis) minutus, Erichthus (Coronis) minutus, and Lysiosquilla (Lysioerichthus) minutus by Brooks (1886) cannot be identified with the adults of any known species until the life-histories of those from the western Atlantic are known much better than they are at present. As material of all of the species previously described from the American Atlantic was available for study, a key to these species has been appended to this paper. The publications by Baiss, Bigelow, Kemp, Lemos de Castro, and Schmitt, listed in the bibliography, are the most important ones for students of American stomatopods; from them, references to most of the scattered literature on the group can be obtained.