13J THE NAUTILUS. 



33]. They are subtriangular in outline, and have spinulose 

 hooks. However, they differ in being slightly oblique, the 

 point of the ventral margin being placed a little posteriorly. 

 80 that tlie anterior portion of the ventral margin is longer 

 than the posterior; the point is also sharper (more project- 

 ing) ; and, finally, they are smaller, and longer than high. 

 While, in U. pictorum, L. and H. are 0.21 mm., in U. doug- 

 lasiae the L. is 0.18 mm. and the H. is 0.15 ram. 



Thus it is perfectly clear that U. douglasiae has the typical 

 anatomical structure of the genus Unio (s. s., type U. pic- 

 torum), and that it cannot be .separated from that genus on 

 anatomical grounds. The question is, whether shell characters 

 permit such a .separation, and in this respect it should be 

 pointed out that the general shape of the shell, the hinge 

 teeth, and other characters are \ery like those of U. pictorum, 

 and that the chief difference is in the beak .sculpture, which 

 is more complex, and covers more of the disk (upon this char- 

 acter Conrad seems to have relied when he created Nodu- 

 laria. But when we compare other species, for instance the 

 European U. tumidus Retz., we see that in all these species 

 the beak sculpture is of the same general type, that is to say, 

 of the zic-zac pattern, and that U. douglasiae represents the 

 most extreme development of this, while U. pictorum has it in 

 much obliterated condition, and U. tumidus is intermediate 

 between these to a degree. Thus there is only a difference in 

 the degree of development, and it should also be bom in mind 

 that even in U. douglasiae the beak .sculpture varies a good 

 deal (see the account given by Haas of the various forms of 

 douglasiae) . 



Consequently we cannot escape the conclusion that U. doug- 

 lasiae is a true Unio in all respects, and that it should stand 

 as Unio douglasiae Griffith & Pidgeon. Nodularia Conrad, 

 with douglasiae as type, becomes then a synonym of Unio 

 Retzius. 



This, probably, refers to those species which are related to 

 douglasiae, that is to say, preeminently to all those associated 

 by Haas in the "group of N. douglasiae.'' It remains to be 

 seen what should become of the other species of Nodularia in 



