Mesolamprops Given, 1964: 288.
Type species. Mesolamprops bispinosus Given, 1964, by original designation.
Diagnosis. Carapace not dorsoventrally flattened, marginal carina absent, eyelobe present, pseudorostral lobes short, blunt. Antennule flagella well developed, accessory flagellum at least 0.5 length of main flagellum. Mandible navicular. Maxillule palp with 2 setae. Pereopod 1 slender, basis as long as all other articles together. Pereopod 5 longer than pereopod 4 basis. Telson long, at least 2/3 length of uropod peduncles. Uropod endopod of 3 articles. Uropod exopod article 1 at least 0.5 length of article 2. Female with fully developed exopod on pereopod 2, rudimentary exopods on pereopods 3–4, without pleopods. Male with long antenna, extending past pereon, without penial lobes, with 2 pairs of pleopods.
Species. Mesolamprops abyssalis Reyss, 1978a, M. bispinosus Given, 1964, M. denticulatus Ledoyer, 1988, M. dillonensis Gladfelter, 1975, M. hartleyi Shalla & Bishop, 2007, M. japonicus ( Zimmer, 1937).
Remarks. Given’s original description (1964) of the genus included reduced pleopods in the male, in which the exopod, or lateral ramus, which is normally bi–articulate is reduced to a small projection. However, subsequent papers ( Gladfelter 1975; Reyss 1978a; Ledoyer 1983) included species that possess two pairs of pleopods, but the state of the pleopods varies from that described by Given (1964). In Mesolamprops dillonensis, the pleopod is illustrated with equal rami, with a bi–articulate lateral ramus and a uni-articulate medial ramus. In M. abyssalis the pleopod is illustrated with 2 equally small uni-articulate rami, and in M. denticulatus the pleopod is illustrated with unequal rami, and the shorter lateral ramus is bi-articulate. The pleopod was not illustrated for M. hartleyi by Shalla & Bishop (2007). The only species with a pleopod that is like that of M. bispinosus is M. japonicus. In the additional description of M. japonicus provided by Tsareva (1999), it is clearly illustrated with a pleopod in which the lateral ramus is reduced to a small projection.
The species described by Gamô (1999) as Mesolamprops bacescui is clearly a member of the Diastylidae, as it has long articles in the male antennal flagellum, two pairs of pleopods without the medial process in the adult male, two terminal setae on the telson in both males and females, and distinct pre and post-anal sections of the telson. In comparison, all male lampropids with fully developed pleopods have a process on the medial ramus, short articles in the adult male antenna flagellum, no obvious pre and post-anal division of the telson, and three or more terminal setae on the telson. Thus, this species is hereby referred to Diastylis, as Diastylis bacescui (Gamô 1999) comb. nov.