780 THE VOYAGE OF H.M.S. CHALLENGER. 



while Palsemon serratus was described and figured under the name of Astacus serratus, 

 in 1770, by Pennant, in his British Zoology, vol. iv. pi. xvi. fig. 28. 



This well-known European form, Palsemon serratus, has ever since been accepted 

 as typical of Fabricius' genus by Cuvier, Leach (1817), Desmarest (1825), Milne- 

 Edwards (1837), and Bell (1853); and Leach, Desmarest, and Bell have, besides their 

 descriptions, given accurate figures of the animal. 



Its chief characteristics are as follows : — A laterally compressed rostrum, serrate on 

 the upper and lower margins ; the dorsal surface of the carapace not carinated posterior 

 to the frontal crest ; the frontal margin armed with two teeth, one corresponding with 

 the first pair of antennae, and the other, close behind the frontal margin, corresponding 

 with the second pair of antennae ; the rest of the carapace smooth, the lateral margins 

 being strengthened by a longitudinal rib, and the posterior margin laterally inserted 

 beneath the coxal plates of the first somite of the pleon. 



This description has been drawn up from a British specimen, and compared with the 

 type in the British Museum, and it corresponds with the figures of Leach, Desmarest, 

 and Bell. 



I have endeavoured to be as accurate as possible in the diagnosis of this genus, 

 because Dr. Stimpson 1 describes the Palsemon of Fabricius as " Carapax spina hepatica 

 armata." This description corresponds with Palsemon carcinus, Fabricius, but not 

 with either Palsemon serratus .or Palsemon squilla. The figures of Leach, Desmarest, 

 and Bell distinctly show the two marginal teeth, and Mdne-Edwards, in his description 

 of his first division of Palsemon, places the two marine species just mentioned under 

 it because " they are armed on the anterior border of the carapace on each side with 

 two teeth, one above, the other below the insertion of the second (externes) pair of 

 antennae." 



Surely all this is sufficiently clear in description and priority to settle that the 

 typical forms of the genus are Palsemon squilla and Palsemon serratus. 



By the expression " species omnes fiuvicolse," it would appear that Stimpson 

 intended to confine the genus to those fresh-water forms that have been found in 

 many of the rivers, lakes, and mountain streams in tropical regions, and which Milne- 

 Edwards has arranged in his second division of the genus, having the frontal margin of 

 the carapace armed with a single tooth, but with a second tooth posterior to it in the 

 same horizontal line. 



Dr. Stimpson altogether excludes the typical species, or those on which the earlier 

 carcinologists founded the genus, but transfers them to the genus Leander, which was 

 proposed by Eugene Desmarest on features that are not of the slightest specific value, 

 namely, on the dorsal surface of the pleon being strongly curved, or, to quote his own 

 words : — " L abdomen est tres grand, et se retrecit graduellement vers le bout ; sa 



1 Proc. Acad. Nat. Sci. Philad., p. 110, January 1860. 



