1878.] AND DISTRIBUTION OF THE CRAYFISH1CS. ~63 



The Branchial formula o/Astacus fluviatilis '. 



Somites „ , ArthrobrancbisB. „. 



and their P°do- , Pleuro- 



appendages. bl ' a » chi *- Anterior. Posterior. branchiae. 



VII 0(ej>) = 0(ep) 



VIII 1 1 () = 2 



IX 1 1 l 0=3 



X 1 1 l o=3 



XI 1 1 l l)=3 



XII 1 1 1 r = 3 + r 



XIII Ill r = 3 + r 



XIV 1=1 



6 + ep+6 +5 + l+2/-=18 + ep + 2/- 



"ep" here signifies a podobranchia which has lost its branchial 

 filaments and become completely metamorphosed into an epipodite, 

 while /• indicates that a rudiment of a branchia exists. 



It will be observed that, in this species of Crayfish, no somite 

 possesses its hypothetically full complement of branchiae except XII. 

 and XIII. ; and even in them the pleurobranchise are rudimentary. 

 The representatives of eleven possible branchiae are altogether 

 wanting. 



2. The Branchice q/*Cambarus. 



The principal distinction between this genus and Astacus, as it 

 was established by Erichson, lies in the absence of the single pleuro- 

 branchia of the latter, and the consequent reduction of the number 

 of the branchiae to seventeen on each side. 



In his elaborate monograph of the genus, Dr. Hagen observes, 

 " But there is also another difference, not before noticed 2 . In 

 Astacus each pair of gills, except the single one on the fifth set of 

 legs, has a broad deeply-folded membrane, closely fixed behind the 

 most external gill-lobe. In Cambarus, this membrane is always 

 wanting in the gills of the fourth pair of legs, but exists, as in 

 Astacus, in all the others. 



" In the true Astacus, all the gills with a folded membrane behind 

 have a basal external bundle of shorter but broader and irregularly 

 placed gill-tubes ; these are never to be found in Cambarus." 



In a species of Cambarus from Guatemala, of which a number of 

 specimens have been presented to the British Museum by Mr. 

 Salvin 3 , I find Dr. Hagen's first remark fully borne out. The last 



1 In this, as in all other cases, it is to be understood that the branchial 

 formula gives the branchia? of only one side of the body, and that the total 

 number of branchire is therefore double that given in the formula. 



2 Dr. Hagen appears to have overlooked De Haan's definition of the distinc- 

 tive characters of the American Crayfishes known to him : — 



" Branchias 17 ; nulla supra pedes quintos; externa supra quartos tantum e 

 tubulis Uteris, externa} supra sequentes infra e tubulis, supra e laminis tuber- 

 culatis compositas " (Fauna Japonica, Crustacea, p. 164). 



3 Mr. Salvin informs me that they were obtained in a river near Coban, 

 in Vera Paz, at an elevation of about 4300 feet above the sea. 



