A new species of crayfish, Orconectes (Crockerinus) alluvius, is described from headwater streams of the Crawford Upland and Mitchell Plain in southwestern Indiana. It occurs in small- to medium-sized sandy streams associated with leaf litter and woody debris. Orconectes (C.) alluvius can be distinguished from other species in the subgenus Crockerinus Fitzpatrick, 1987 by a combination of the following characters. First pleopod of form I male with weakly developed shoulder on cephalic surface at base of central projection; central projection corneous, constituting 18.9–32.5% (X̄ = 24.5%, n = 59, SD = 3.39) of total length of first pleopod, straight for most of length with a slight lateral curve at tip; cephalic margins of central projection feature impressed longitudinal striations distally on both mesial and lateral margins; mesial projection non-corneous with a slight outward bend near middle, ending in a flattened tip that is wider than the tip of the central projection; length of mesial projection subequal to length of central projection. The areola is wide, ratio 3.7–6.8 times of width to areola length, abdomen shorter than the thorax, and narrow chelae width. Mesial palm of chelae with three rows.
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