676 General Notes. [August, 



seen no record of the occurrence of the Monodontus in localities 

 inhabited by the Margaritifera, it may fairly be said to be its rep- 

 resentative over the region, or a large part of it, where the latter 

 is wanting. In fact, the suggestion of close relationship, and dif- 

 ferences due to undetermined causes, make a thorough study of 

 these two species, by competent histologists, a matter of very 

 great interest. This is only one, and that not a specially 

 emphatic case of equivalents, some of which I hope to cite in 

 part 111 of this discussion, and every phase of it seems to me to 

 look in the direction pointed out in part II. It remains to say 

 that this shell has a close analogue, and a southern one at that, 

 in Europe, this being the Unio sinuatus Lam., a shell found in the 

 Rhine, Meuse, Seine, Rhone, Loire, Garonne, Charente, Adour, 

 Dordogne, Tarn, etc., but of whose northern distribution there 

 seems to be no record. In his Mollusques de France, p. 634, the 

 Abbe Dupuy makes the following remarks on this species : 



"Cette espece si remarquable et dont les caracteres sont si 

 tranches, a ete confondue par presque tous les auteurs francais, 

 apres Draparnaud, avec ia \l<i , ■ tr^r /Lam., qui en 



differe essentiellement puisqu' elle n' a pas les lamelles posteri- 

 eures qui en ont fait faire un genre particulier par plusieurs auteurs. 

 L'erreur est venue, chez les auteurs francais de ce qu'ils n'avient 

 pas en connaissance d echantillons authentiques d' Unio margan- 

 ti '-ferns Retz. [My a man* until era Linn.). D'un autre cote, les 

 etrangcrs ont donne le nom d' U. sinuatus a une grosse var. sinuee 

 inferieurement du veritable Unio margaritiferus Retz , parce qu'ils 

 ne connaissaient pas non le veritable U. sinuatus Lamk. 



I wish to add, that having been favored with numerous speci- 

 mens of both these species, from authentic localities, I can see 

 how these shells may have easily seemed to collectors to be very 

 similar. The fact that a species so closely allied to the J/ mar- 

 garitifera, and yet so evidently distinct from it, exists both in 

 Europe and America, is one of considerable interest, and when 

 taken in connection with other associated facts, becomes some- 

 what suggestive. 



the geographical distribution and variation of the Unionidse, and 

 the exchange of specimens — A. G Wethcrby. 



Nomenclature of external parts of Arthropoda.— The 

 following terms have been devised for convenience in anatomical 

 and systematic work on the Arthropoda, and are submitted 

 for the judgment of naturalists. We have adopted them in a 

 forthcoming monograph of N. A. Phyllopoda now in the press. 



The term arthromere, originally employed in the author's 

 " Guide to the Study of Insects " in i860, is now restricted to the 

 body-segments of Arthropods, the term' zonite or somite being 

 used for the body-segments of worms, as well as Arthropods. 

 The *' head," " thorax," and "abdomen" are termed respectively 



