140 THE NAUTILUS. 



(in the outer gill closer than in the inner). Tiie septa show, in the 

 sterile female, in both gills the characteristic folded epithelium. 

 When gravid, all four gills swell but only moderately, and the edges 

 always remain sharp, and do not possess the faculty of distending. 

 The water tubes (ovisacs) remain simple, and in each of them the 

 eggs form a subcylindrical, only slightly compressed, mass, a 

 placenta, which, however, is not very solid, the mutual adliesion of 

 the eggs being rather slight. 



With the exception of the two features mentioned above, Parreysia 

 is thus identical, in its anatomy, with Quadrula. If we add to these 

 two differences the radial sculpture of the beaks, we would have 

 good generic cliaracters. But the genus, at any rate the species 

 wyneyungaensis which stands close to the type species of the genus, 

 corrugata, should be removed from the subfamily HyriincB, and 

 placed into the subfamily Unionince (Nautilus, 23, '10 p. llG),by 

 the side of, and close to Quadrula. The definition of the TJidonince 

 should be slightly modified with regard to the supraanal opening and 

 the inner lamina of the inner gills, so as to include Parreysia, but 

 these modifications are irrelevant. 



This, of course, means that I do not attribute any significance to 

 the beak sculpture, but I think I am fully justified in this. If we 

 make radial beak sculpture the prime character of the Hyriince, we 

 would be compelled not only to unite the Afro-Asiatic forms, like 

 Parreysia, vi\i\\ the South American Hyriince, which, I believe, is a 

 great mistake, but we would also have to unite with the Hyriince a 

 number of living North American shells. For we must not forget 

 (and this is a fact generally overlooked) that we have radial (zig- 

 zag) leak sculpture in a number of species of Qiiadrida. I name the 

 following : Quodrula (^Rotundarid^ tuhercidata (Raf.), Qu. infucata 

 (Conr.) and kleiniana (Lea), Qu. lachrymosa (Lea), forsheyi (Lea), 

 speciosa (Lea), apiculata (Say). Also in Qu. cylindrica (Say) 

 traces may be seen. This zig-zag radial sculpture is best developed 

 in the species first named ; in the others it goes generally as 

 " double-looped," but, in my opinion, the so-called " double-looped " 

 sculpture is but the last remnant of the zig-zag sculpture. 



Furthermore, radial beak sculpture is a quite frequent feature in 

 North American fossil Unionidce from the Mesozoic era. If we look 

 at the beak sculpture of Unio holmesianns White (see : 3 Ann. Rep. 

 U. S. Geol. Surv. '83, pi. 16, fig. 2-6) from the Laramie of southera 



