ASTACIDEA. 523 



much longer than the base of these organs, and, moreover, the species, 

 unlike those of Nephrops, are fresh-water. Through Nephrops and 

 Paranephrops the Astacoidea graduate towards the Caridea. 



The genera of living Astacidse adopted, will then be as follows : — 



1. Manus crassse et latse, marginibus arcuatse, stiperficie convexse (Astacin^s). 



A. Branchiae 19. Segmentum thoracis ultimum non mobile. — Species marinse. 



G-. 1. Homarus, Edwards. — Rostrum tenue, utrinque paucidentatum. Squama 

 basalis antennarum externarum perbrevis. 



B. Branchise 17-18. Segmentum thoracis ultimum mobile. Rostrum integrum vel utrinque 

 unidentatum. — Species fluviales. 



Gr. 2. Astacoides, Guerin. — Segmentum abdominis maris lmum appendieibus 

 carens. — Subgen. Astacoides, branckiis 18 ; Cheraps. (i7nc/i.)branchiisnuinerol7. 



Gr. 3. Astacus. — Segmentum abdominis maris lmum appendieibus instructum. 

 Subgen. Astacus branckiis 18; Cambarus, (Erich.) branchiis numero 17. 



2. Manus prismaticse, lateribus fere rectss (NephropiNuE). 



G. 4. Nephrops, Leach. — Rostrum utrinque dentatum vel spinosum. Squama 

 basalis antennarum externarum basi vix longiores. — Species marinae. 



G-. 5. Paranephrops, White. — Rostrum ac in Nephrope. Squama basalis anten- 

 narum externarum basi dimidio longiores. — Species fluviales. 

 / 



In some recent English works, the name Potamobius has been sub- 

 stituted for Astacus, and Astacus for Homarus, following Leach, who, 

 in 1819, made this arrangement of the species at that time referred 

 to Astacus. In the nomenclature generally adopted, as is well known, 

 the system of Edwards is followed, who, in the second volume of his 

 Crustaces, subdivided the old genus Astacus in the same manner as 

 Leach, but gave the name Homarus to the marine species, and retained 

 Astacus for the rest of the genus. In the Catalogue of Crustacea of 

 the British Museum, published in 1847, the names of Edwards are 

 used, while in the Catalogue of British species, published in 1850, 

 Leach's nomenclature is adopted. 



Leach has undoubted priority, and exhibited his usual discrimi- 

 nation in proposing the subdivision of the old genus. But the appro- 

 priating of the name Astacus to the single marine species, violates 



