1873.] J. Wood-Mason — On N'ejpliropsis Stewarti. 43 



The last abdominal somite is immoveably united to that which precedes 

 it as in Nephrons and the common Lobster ;* and the sternum is linear as in 

 the Astacidce generally. 



Fost-ahdomen. — The post-abdomen is gradually attenuated to the 

 extremity of the telson. The appendages of its first somite are as complete- 

 ly rudimentary as they d^vQ iw i\\Q ievci2i\Q oi JSTeplu^ops JS'orvegicus ;\ those 

 which follow are long and slender, their foliaceous branches being very nar- 

 row, produced to a sharp point, and fringed with excessively long cilia. All 

 the terga are covered with minute rounded tubercles, and present at their 

 anterior ends, just behind the tergal facets, a broad smooth transverse groove 

 with its hinder margin convex backwards. 



The pleuron of the first somite is precisely similar to that of Neplirops 

 Norvegicus, but those of the remaining somites are even more acutely trian- 

 gular than in that species, and have their margins denticulate and furnished 

 with a fringe of long cilia. In all the somites, with the single exception of 

 the first, the tergal and pleural regions are* most sharply defined as such, the 

 former not curving continuously with the latter but terminating abruptly 

 at the level of the ventral chords in a line convex outwards ; so that, if a 

 somite were detached, deprived of its ventral chord and flattened out on the 

 table with its dorsal surface uppermost, the imaginary continuation from 

 pleuron to pleuron of the plane in which these pleura laid, would pass below 

 that of the surface of the tergum. 



The ' swimmeref constituted as in all other Macrurous Crustacea by 

 the highly modified and backwardly placed appendages of the last postabdo- 

 minal somite and by the ' telson^ differs in no particular of more than speci- 

 fic value from that of Neplirops ; the mesial element, or telson, is longer in 

 proportion to its breadth, its greatest breadth, being a transverse line separat- 

 ing its anterior from its middle third, and not at the base as in Neplirops, is 

 slightl}^ more truncate posteriorly, and the oblique rounded elevations, that 

 gradually narrow^ as they pass backwards into the spines at its postero- 



* On characters furnished by the claws alone Dana artificially divides the recog- 

 nized genera of Astacidce into two groups, typified respectively by Astacus and Ne- 

 phrops ; the first of these is further subdivided according to the number of the 

 branchiae and the mobility or immobility of the last abdominal somite. But no men- 

 tion is made of the fact that this is firmly fixed in NepUrops too. If ParanephropSf 

 a genus including only freshwater forms, should turn out to have a mobile last abdo- 

 minal somite, then we shall have this curious fact presented to us, viz., that all 

 those members of the family Astacidce which live in freshwater or are terrestrial (En- 

 gceus) have this somite moveably united by membrane only to that which precedes, 

 while those of them that are marine have it fixedly united to the rest of the sternum. 



t The ventral plates of the 2nd, 3rd and 4th postabdominal somites in the males 

 of 'Neplirops Norvegicus have an erect spine in the middle line, but the females exhibit 

 no trace of such. 



