Poorly developed fins, dorsal fins in contact in mature adults, blue-green back, yellow flanks, white ventral portion (Ref. 2163). Diagnostic features: Adults: 8.6-17.0 cm TL. Body proportions, as percentage of TL (based on 16 specimens measuring 10.6-13.3 cm TL): prebranchial length, 10.9-12.6; branchial length, 7.5-9.9; trunk length, 44.4-53.3; tail length, 25.4-31.4; eye length, 2.3-2.9; disc length, 5.2-7.0. Urogenital papilla length, as a percentage of branchial length, in ten spawning males measuring 11.1-14.0 cm TL, 21.4-52.6. Trunk myomeres, 60-65. Dentition: supraoral lamina, 2 unicuspid teeth; infraoral lamina, 7-9 teeth, usually all unicuspid, but occasionally one lateralmost bicuspid; 3 endolaterals on each side; endolateral formula, typically 2-3-2; 2 rows of anterials; first row of anterials, 9 unicuspid teeth; exolaterals absent; posterials absent; transverse lingual lamina, 9-13 unicuspid teeth, the median one enlarged; longitudinal lingual laminae each with about 9 unicuspid teeth. Velar tentacles, 4-6, with tubercles. Body coloration (preserved), dorsal and lateral aspects grayish, ventral surface whitish, dorsal fins and caudal fin yellowish. Some specimens display a prominent dark blotch at the apex of the second dorsal fin. The iris is yellow. Lateral line neuromasts unpigmented. Unpigmented caudal fin except for spawning individuals in which it is "weakly peppered". Caudal fin shape, spade-like (Ref. 89241). Sigmund Freud (Ref. 72450, 72451 ) described the development of the central nervous system of brook lamprey.