29 April 1988 PROC. BIOL. SOC. WASH. 101(1), 1988, pp. 221-226 TULUMELLA UNIDENS, A NEW GENUS AND SPECIES OF THERMOSBAENACEAN CRUSTACEAN FROM THE YUCATAN PENINSULA, MEXICO Thomas E. Bowman and Thomas M. Iliffe Abstract. - Tulumella, new genus, the sixth genus of Thermosbaenacea, with type species T. unidens, new species, is reported from Najaron Cave, near TulGm, Quintana Roo, Mexico. It is characterized by having small non-func- tional eyestalks, a scale on antenna 2, mandibles that may lack a lacinia mobilis and have reduced molars, a biramous pereopod 1, and a reduced pleopod 2. The family Monodellidae is recognized as valid, and a key is given to the families and genera of Thermosbaenacea. In November 1986 the second author made collections in nine caves and cenotes on the Yucatan Peninsula, Mexico, mostly in the vicinity of the ancient Mayan city of TulGm. From one of these caves he ob- tained three specimens of the new ther- mosbaenacean described below. Order Thermosbaenacea Monod, 1927 Family Monodellidae Taramelli, 1954 Telson separate from pleonite 6. Seven pairs of pereopods present. Maxilliped of 6 with or without endopod. Tulumella, new genus Diagnosis. -Small eyestalks present, without visual elements. Antenna 1 long, with up to 18 flagellar segments. Antenna 2 with scale (exopod). Right or both mandi- bles without lacinia mobilis; molar slender, cylindrical; chewing surface divided into a few slender spiniform teeth. Maxilla 2 with 2 broad basal endites armed with spoon- shaped setae; exopod setose, inserted well lateral to endopod. Maxilliped with broad endopod and exopod, both with several marginal setae. Pereopod 1 biramous, en- dopod 5-segmented (including basis); pe- reopods 2-7 with 2-segmented exopod and 6-segmented endopod. Pleopod 1 a small pyriform unarticulated lobe; pleopod 2 ar- ticulated, elongate. Uropod exopod slightly longer than endopod; 1st segment slightly longer than 2nd. Type species. - Tulumella unidens, new species. Etymology.-From the ancient Mayan city of Tulum, with the diminutive Latin suffix "ella." Tulumella unidens, new species Figs. 1-2 Material. -Mexico: Quintana Roo, near ruins of Mayan city of TulGm: Najaron (Na- haron) Cave, leg. T. M. Iliffe 11 Nov 1986 (collection no. 86-106), 3 specimens: ho- lotype, 2.9 mm, USNM 233394; paratypes, 3.0 and 1.8 mm, USNM 233395. Etymology. -From the Latin "uni-" (one) + "dens" (tooth), referring to the lack of a lacinia mobilis in both mandibles. Diagnosis. -As for the genus. Description. -Length up to about 3 mm (the 3 mm paratype is in 2 pieces; hence its measurement is approximate). Carapace covering pereonites 1-6. Eyestalks oval, close together, partly covered by carapace.