DECAPODA. 49 



Pactolus, Leach. 



The four or six anterior feet simple, or without forceps. The in- 

 ternal extremity of the penultimate joint of the four posterior ones 

 is prolonged into a tooth, forming with the last joint a forceps or 

 didactyle hand. The form of the shell is that of the Leptopodia:, 

 and the tail presents the same number of segments; but the feet are 

 much shorter; those of the third pair were wanting in the individual 

 which served as the type of this section(l). 



Lithodes, Lat. 



The Lithodes, as to the form of the first eight pairs of feet, re- 

 semble the other Trigona; their length, however, seems progressively 

 to increase from the second to the fourth, but the two last are very 

 small, bent, but slightly visible, beardless, and apparently useless. 

 The tail is membranous with three crustaceous and transverse 

 spaces on the sides and another on the end, representing the seg- 

 mentary divisions. The eyes are approximated inferiorly. The ex- 

 ternal foot-jaws are elongated and salient, and the shell is triangu- 

 lar, extremely spinous and terminated anteriorly by a dentated point. 

 These Crustacea are peculiar to the Arctic Seas(2). 



Our sixth section, that of the Cryptopoda(S) consists of Brachyura 

 remarkable for a vaulted projection of the posterior extremities of 

 their shell under which their feet, the two anterior or the claws ex- 

 cepted, can be completely retracted and concealed. The shell is 

 nearly semi-circular or triangular. The superior edge of the forceps 

 is more or less elevated and notched in the manner of a crest. In 

 those species where they are largest, they cover the anterior part of 

 their body, and hence the name of Coq de mer (Sea Cock), and 

 Crabe hontenx (Bashful Crab), which have been given to some of 

 them. One sub-genus of this section, that of JEthra being closely 

 allied by other characters with the Parthenopes of Fabricius, the 

 first sub-genus of the preceding section, it follows, in a natural order 



(1) Pactolus Boscii, Leach, Zool. Misc., lxviii. 



(2) Cancer maja. L.; Parthenope rnaja, Fab.; Inachus maja, Id.; Lithodes arc- 

 tica, Leach, Malac. Brit., xxiv. See also the Maja camptschensis, Tiles., Mem. 

 Acad. St. Petersb., 1812, V, VI. 



(3) Several of the Arcuata, such as the Hepati, Mul'siae, Matutse, among the 

 swimmers, have a crested forceps, and seem to be naturally allied to the Crypto- 

 poda, so that this section should be placed higher in the scale. The same obser- 

 vation applies to the last one, or that of the Notopoda, for some of them approach 

 the Arcuata, and others the Orbiculata and the Trigona. 



Vol. III. G 



