ASTACIDEA. 521 



wholly membranous, and the caudal segment undivided; in a second 

 (Cheraps), these feet are calcareous, the caudal segment is membra- 

 nous in its posterior segment, and the legs of the fifth pair are without 

 branchiae ; in a third (Engaeus), the legs of the fifth pair are furnished 

 with branchiae, the caudal extremity is as in Astacus. 



Erichson adds in his characteristic of these subgenera, that in 

 all of them, excepting Engasus, the outer antennae are situated exterior 

 to the inner antennae, while in the genus just mentioned, they are 

 under the inner antennae, a character in which the several species of 

 Astaci widely differ, yet restricted, without sufficient study, we think, 

 to the Engsei alone. 



The subgenus Astacaidea was first established by Gu^rin, in 1839 

 (Rev. Zool., p. 109), for the A. madagascariensis of Edwards, which 

 he described the same year, under the name Astacoides Goudotii. 

 Guerin erred in overlooking the small basal scale of the outer antennae, 

 and based his genus on its supposed absence. 



The year preceding the publication of Erichson' s Memoir, J. E. 

 Gray, Esq., published a paper on some Astaci from New Holland, in 

 an Appendix to Eyre's Discoveries in Central Australia,* in which he 

 suggests that the genus Astacus may be divided into three sections, 

 distinguished as follows : 1. Caudal segment bipartite, and each part 

 hard and calcareous, as in A. fiuviatilis, A. Bartonii, &c. ; 2. Caudal 

 segment not divided, calcareous quite to its extremity, as in A. mada- 

 gascariensis, Edwards,f and Franklinii, Gray; 3. Caudal segment 

 entire, or only slightly divided near the middle of each margin, with 

 the texture thin and flexible posteriorly, as in A. 5-carinatw, Gray, 

 A. 2-carinatus, Gray, and A. cliilensis, Edwards. 



In laying down these sections, the caudal segments and appendages 

 afford the principal characters, while the existence or not of appen- 

 dages to the first abdominal segment, and the presence or not of a 

 branchia at the base of the fifth pair of legs, points seized upon by 

 Erichson, are overlooked by Gray. 



With regard to the absence of the branchia from the legs of the fifth 

 pair in the American Astaci, the fact was first noticed by De Haan, 

 who examined particularly the A. Bartonii, and A. affinis, and men- 



* Journals of Expeditions of Discovery into Central Australia, in the years 1840, 

 1841, by Edward John Eyre. 2 vols., 8vo., London, 1845. Appendix, p. 410. 



j- Edwards says, that in the Maduyascarienais the lateral appendages of the tail are 

 semicorneous towards their posterior border. Archives du Museum d'Hist. Nat. 1839. 



131 



