REVISION OF THE ASTACID^E. 



The family Astacidae, in a strict sense, is equivalent to the genus Asta- 

 cus, as limited by Milne Edwards in 1837.* Thus restricted, it includes only 

 fresh-water species, the crayfishes proper, — animals closely related to the 

 marine family of lobsters, or Homaridae (Homarus, Nephrops, Nephropsis, 

 Enoplometopus, Phoberus), from which they are distinguished by having 

 the last segment of the thorax movably articulated with the preceding 

 segment, and by having the podobranchise (or gills borne on the thoracic 

 limbs) closely united to the epipodites (or external branches of the limbs), 

 to the more or less complete suppression of the lamellar portion of the 

 epipodite. On the other hand, the Astacidaa lead, through such forms as 

 Thaumastocheles f and Calocaris, to the fossorial Thalassinidae. 



Thus limited, the Astacidae are Macrourous Decapod Crustacea, with 

 carapace produced into a rostrum in front, and divided by a transverse 

 groove (cervical groove); narrow thoracic sterna; posterior thoracic somite 

 united to the preceding somite by a movable joint ; second to sixth abdomi- 

 nal somites with a broad descending lateral plate (pleuron) on each side, 

 which protects the abdominal appendages; antennulcs terminated by two 

 filaments; antennae furnished witli a movable external scale or squame;t 

 first pair of legs much enlarged, chelate; second and third pairs of legs with 

 small chehe; last two pairs of legs not chelate; first pair of abdominal 



* llistoirc Naturelle des Crustaces, Tom. II. p. 329. 

 • f A genus founded bj Wood-Mason (Proo. Asiatic Soc. Bengal, 1S7-1-, p. 151) for the reception ot 

 dstacus zateucus Willemoes-Sulim, brought np bj the "Challenger" from :i depth of 450 fathoms, near 



S brero Island, West Indies. Sec Trans. Linn. Soc. London, 2d Series, Vol. I., Zool., pp. tS 50, 



PI, £. fig. 1. t m 1 1 i hr structure of the gills is known, it is impossible to say whether, fr the sum of its 



characters, Thaumastocheles should In 1 placed among the Astacida; or among the Thalassinidte. 

 % The antennal scale is rerj small in dstacoidet Madagascariensis And, ei \1. Edw. 



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