of Recent Spiciea oj Limuliis. 261 



Sixth prosoiiiatic a}ipen(la<jo with movable spur at the 

 distal end of tlie undersido ot' the foiuth seo^meiit. Postanal 

 p])ine triangular in section, carinate above, liij^htly convex or 

 flattish, not excavated below. Clasper of male hemichelate, 

 the immovable digit reduced to a short, thick, thumb-like 

 ])rominence. 



Xiphosiira polyphemus (Linn.). 



Monoculus pohiphrmtis, Linn. Syst. Nat. ed. x. p. 634 (1758) (in part.) ; 



id. Mas. Ulr. Iie<r. p. 4()0 (17G4) (senPii stricto). 

 Limiiliis //it/as, Miiller, Entomostvaca, p. 12(5 (178o) (in part.). 

 Limulus cijclojjs, Fahr. Ent. Syst. ii. p. 488 (1793) (by tradition). 

 Limnlus occidentiiUs, Lamarck, Syst. Anim. sans Vert. p. ](J8 (1801) 



(nonion nudum) ; id. ed. 2, vol. v. p. 147 (1818). 

 Lhmthis albus, lio^c, Hist. Nat. Crust, ii. p. 237 (1802) (according to 



Desmarest). 

 Limnlm poli/plwmus, Latreille, Desmarest, Leacb, ^'an der lloeven, 



INIilne-Julwards, and subsequent authors. 

 Limulus Souh-rhii, Leacb, Zool. ^lisc. ii. p. 84. 

 Limulns amcricanus, Leach, IJict. Sci. Nat. p. 537 (1819). 



Disirihution. This king-crab is said to range from the 

 coast of Maine to Yucatan in the Gulf of Mexico. Hardly 

 likely is it that more than one species is involved; but con- 

 .^idering the different conditions of water-temperature to which 

 tlie species must be subjected, it is probable that some inter- 

 esting results would follow a comparison of a series of 

 specimens from Cape Cod with a series from Yucatan. 



Subfamily Tachtplein^, nov. 

 (PI. V. figs. B, C, D, and PI. VI. fig. 6.) 



Genital operculum more specialized than in Xiphosura, the 

 terminal segment of the inner branch, which is retained by 

 that genu.", suppressed, probably by fusion with the second 

 segment, the two being represented by a triangular plate, 

 Iroad at its proximal, narrow at its distal end, movably 

 jointed to its fellow of the opposite side internally, and 

 \o the terminal segment of the outer branch externally, by a 

 sutural union. Thus the distal segments of the outer 

 branches aie terminally approximated in the middle line and 

 not separated by a pair ot movable segments. 



Entosternum with a single pair of antero-lateral processes. 

 Second and third pairs of prosomatic appendages modified 

 as claspers in the male. 



Opisihosoma less vaulted, more markedly hexagonal owin"- 

 to the greater prominence of the lateral angle which lies near 

 the middle of the lateral border, making its spiniferous and 

 non-spiniferous parts subequal ; the latter not abruptly bent 

 downwaixls posteriorly, the area behind its spike, which lies, 



