Key to adult females of Mucroseius (based on Lindquist and Wu 1991)

1. Tarsus IV with 2 macrosetaeı pd2 and ad2 ı distinguished by nearly perpendicular orientation and greater length from other tarsal setae........................................................ 2 – Tarsus IV with 1 macrosetaı only pd2 readily distinguished as such ............................ 7

2. Fixed chela with broadly rounded distal flange; tibia III with 9 setae ( pl2 present); dorsal shield ornamented with conspicuous little ridges and reticula over entire surface ................................................................................................................................ M. aciculatus

– Fixed chela with slenderı bluntly pointed distal flangeı or flange absent; tibia III with 8 setae ( pl2 absent); dorsal shield smooth or reticulateı not carinulate ...................... 3

3. Fixed chela without distal flange ............................... ............................. M. insolitus sp. nov. – Fixed chela with slender distal flange.......................................................................................... 4

4. Epistome weakly triramousı its median process broadı untapered and conspicuously serratedı and its lateral processes weakly projected and conspicuously serrated; dorsal shield with setae s4 nearly as long or longer than caudal setae Z5; leg IV with anterior macroseta subequallyı or within 15 μm as long ası posterior macroseta .. M. disparisetus

– Epistome clearly triramousı its medial process tapering to a slightly serrated or smooth pointı and its lateral processes well projected and sparsely serrated; dorsal shield with setae s4 usually not more than 0.8 as long as caudal setae Z5 (some M. algonquian excepted); leg IV with anterior macroseta clearly (by 20 – 30 μm) shorter than posterior macroseta ........................................................................................ 5

5. Dorsal shield smooth over most of surfaceı with 7 – 9 pairs of marginal setae ( r2–r6, R 1– R 2 ı sometimes R 3– R 4) inserted on its lateral edges; epigynal shield reticula outline 12 – 16 finely elongate cells between bases of setae st5; sternal shield with posterior margin deeply excavated to level of setae st2............................... M. nipponensis

– Dorsal shield smooth or reticulated over most of surfaceı with only 2 or 3 pairs of marginal setae ( r2–r3 ı sometimes r4) inserted on its lateral edges; epigynal shield reticula outline 9 – 11 elongate cells between bases of setae st5; sternal shield with posterior margin deeply excavated or not................................................................................ 6

6. Dorsal shield conspicuously reticulate over most or all of surface (sometimes smooth medially from setae j4 to j6 ı and from J1 to J4); sternal shield with posterior margin often deeply excavated to level of setae st2 or poroids iv2 ı occasionally this margin remaining weakly evident at level of setae st3.................................................. M. algonquian

– Dorsal shield mostly unornamentedı smooth over most of surface (lightly reticulated along anterolateral marginsı and sometimes midlaterally between setae j6–J2 and z6–Z2 and caudally); sternal shield with posterior margin gently concave to level of setae st3............................................................ ............................................................ M. monochami

7. Dorsal shield strongly ornamentedı squamateı over entire surface; 8 or 9 pairs of marginal setae ( r2–r6, R 1– R 3 ı sometimes R 4) inserted on lateral edges of dorsal shield; sternal shield with posterior margin deeply excavated to level of setae st2.............. .............. M. squamosus

– Dorsal shield unornamented over most of surface; 3 – 6 pairs of marginal setae ( r2–r4 ı sometimes r5–r6 ı rarely R 1 or R 2) inserted on lateral edges of dorsal shield; sternal shield with posterior margin gently concave to level of setae st3................ ................ 8

8. Anal shield with anterior margin flattened medially; dorsal shield usually lacking one or both setae of pair z3 ı and with setae J1 and J2 extending slightly beyond bases of J2 and J3 ı respectively.................................................................................................. M. californicus

– Anal shield with anterior margin smoothly rounded medially; dorsal shield with both setae of pair z3 presentı and with setae J1 and J2 barely or not quite extending to bases of J2 and J3 ı respectively ............................................................................ M. mexicanus