REVISION OF THE ASTACID^. 



The faniilj' Astacida?, in a strict sense, is equivalent to the genus Asta- 

 cus, as limited by Milne Edwards in 1837.* Thus I'estricted, it includes only 

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

 marine family of lobsters, or Homaridre (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 podobranchia? (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 Astacidoe lead, through such forms as 

 Thaumastochelest and Calocaris, to the fossorial Thalassinidna. 



Thus limited, the Astacidfe are Macrourous Decapod Ci'ustacea, with 

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

 groove (cervical groove); narrow tiioracic 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 ; antennules terminated by two 

 filaments; antennas furnished with a movable external scale or squame;$ 

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

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



* Histoire Naturclle des Cnislaces, Tom. II. p. 329. 



f A genus founded by Wood-Mason (Proc. Asiatic Soc. Bengal, 1874, p. ISl) for the reception of 

 Antaeus zaleurus Willemoes-Sulim, brought up by the "Challenger" from a depth of 450 fatlioias, near 

 Sombrero Island, West Indies. See Trans. Linn. Soc. London, 2d Series, Vol. I., Zool., pp. 48-50, 

 PI. X. fig. 1. Until the structure of the gills is known, it is impossible to say whctlicr, from the sum of its 

 characters, Thaumastochelcs should be placed among tlie A.stacida." or among the Thalassiuidic. 



X The auteunal scale is very small in Aslacoides Mudagascanensis Aud. et M. Edw. 



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