THE NAUTILUS. 127 



Mr. Button's collection at Oakland is notable for its fine series of 

 C^'praeas, including some of thp rarest Pacific coast forms of Trivia. 



The collection of the University contains a number of Cooper's 

 and Carpenter's types, but awaits a new building for its proper 

 display. 



THE ANATOMICAL STRUCTURE OF CERTAIN EXOTIC NAIDEB COM- 

 PARED WITH THAT OF THE NORTH AMERICAN FORMS. 



BY I)K. A. K. OUTMAX. 



(^Concluded from page 120). 



In all other cases tlie diaphragm is comjdete, and extends to or close 

 to the posterior margin ot the mantle, where it separates the anal from 

 the branchial opening. Two types are recognizable, which form as 

 many distinct and fundamentally different groups (families). In the 

 one (^Unionidce of North America and Asia) tlie gills alone form the 

 diaphragm. In the other (South American and African forms, pos- 

 sibly to be called Mutelidce) the diaphragm is formed anteriorly by 

 the gills, but posteriorly by the union of the mantle itself. 



(b) The mantle edges are originally free all around. IJut a ten- 

 dency develops soon to form two distinct openings, the siphons. In 

 Margaritana the most primitive conditions are observed, and the two 

 openings are distinguished only by the development of the papilla;. 

 Closely allied conditions are found in a genus of typical XJnionidce 

 {Rotundaria). Rut the general tendency is, among the Uniomda, 

 not only to draw the mantle edges together by the diaphragm, thus 

 separating anal and branchial, but also to limit the anal above by the 

 junction of the mantle edges, which, however, leaves open above the 

 anal a supra-anal opening, which only in rare instances becomes 

 closed (^Lavipsilis parva). The branchial opening in the Unionidce 

 is never defined anteriorly by a growing together of the mantle mar- 

 gins. (In the higher forms of the UnionidcB, subfamily LmnpsiUnee , 

 special structures develoj) in front of the branchial opening, chiefly 

 in the female, which serve as devices for the aeration of the marsu- 

 pium during the breeding season.) 



In the other family (Mutelida?) the two openings are always sep- 

 arated from one another by a diaphragm formed by the mantle. In 

 some cases the mantle edges are free from the re«t. In other casos 



