24 CURRENT LITERATURE. 



The introduction first gives a brief sketch of the different classifications from 

 that of Lea (1836) to the author's own in 1896. Apart from the work of Lea, 

 Troschel, and Pelseneer, very little attention has heen paid to the anatomy of the 

 group. The author in his earlier paper (Proc. U.S. Nat. Mus., 1896, xviii), placed 

 a great variety of forms under the generic name Unio, but since then from a study 

 of the soft parts, he has come to the conclusion that it would be best to split up this 

 genus, somewhat after the manner in which Pilsbry has treated the old group Helix. 

 This dismemberment is warranted by the following facts. Under ordinary conditions 

 there is but little difterentiation in the soft parts, but at the period when the ova 

 pass into the gills a remarkable change is brought about in these organs. " In the 

 Anodonta cdcntula of Say, short, horizontal ovisacs are developed, which run 

 directly across the animal, and which at maturity break through the outer walls of 

 the outer gills and pass with their young entire into the water. In the forms typified 

 by Unio nnodontoides the young are contained only in very distinct vertical or oblique 

 ovisacs in the hinder part of the outer gills ; in U. crassidens, pidorum, and the like, 

 the embryos fill the entire outer gills, forming thick, smooth pads; in U. mdancorus. 

 trigoims, mnltijdicatus, and allied forms, they occupy all four of the branchiae 

 throughout. In U. phascolus the smooth outer gills begin to be crimped as they are 

 being tilled with embryos, until when full, they become a series of marvelous folds. 

 In U. irroratus several ovisacs in the center of the outer gills grow out to a great 

 length, become filled with young, and are closely coiled. In U. corniUus a few 

 central ovisacs develop so as to project below, in a long, straight flap. In all the 

 South American and Australian Unios, so far as is known, the inner gills alone, as a 

 rule, are filled with young, and this is probably the case with the species of the 

 Ethiopian region and most of those of south-eastern Asia. After the young have 

 passed out into the water the gills of all the species change back into their ordinary 

 condition, and when not gravid there is great similarity in those of most of the 

 species formerly classed as Unios." 



It would seem that these peculiar evanescent characters, assumed when the gills 

 act as marsupia, are quite constant, and further, are concomitant with certain minor 

 shell characters, consequently the author thinks they may be used as a basis for the 

 foundation of genera. 



Von Ihering's discovery in 1893, showing that certain species on hatching from 

 the egg commence life as a glochidium. with a bivalve shell capable of containing 

 the animal, and others as a lasidium, with three segments, the middle one only 

 having a single shell, the former being regarded as members of the Unionidac, 

 and the latter as of the Muididac, is rightly regarded as the most important discovery 

 that has yet been made in the study of the Naiades. Mr. Simpson's researches upon 

 the gills and ovisacs must rank as the second most important, and we trust that at 

 no distant date he will describe these changes in greater detail and illustrate the 

 same. 



In the past it has been claimed by certain malacologists that the Naiades were 

 hermaphrodite, and by others that the sexes were separate, but from the recent 

 careful researches of Sterki, Taylor, Kelly, and others, it would seem that in the 

 more specialised Uiiionidac, viz. those having two forms of shell and the ovisacs 

 situated in the hinder portion of the outer gills, the sexes are always separate ; 

 whilst in the more generalised, viz. those with one form of shell and the embryos 

 occupying the entire gill, the sexes may or may not be separate. 



The author recognises about 1000 species and 82 varieties of Unionidac, 

 comprised in 61 genera, of these 553 species and 55 varieties belong to Nth. 

 America and loi to Sth. America. Of the Mutclidae, 117 species and li varieties 

 are listed comprised in li genera. 



With von Ihering's view that the primitive beak sculpture of the Unionidac was 

 radial, Mr. Simp.son is inclined to agree, further he believes " that the earlier Unios 

 had the young contained in the inner branchial alone, and that there has been a 

 gradual development from these primitive forms with simple, dull-colored, smooth 



