THE NAUTILUS. 41 



marsupial : the septa are more strongly developed, but keep about 

 the same distance from each other as in the male; they are longer in 

 the transversal direction, and possess, at their insertion at the outer 

 lamina of the gill (primary limb) a marked swelling, within which 

 generally the blood vessel is located. 



The structure of the soft parts of this species of Spatha is remark- 

 able for the following six particulars. 



1. The compk'te absence of a supraanal opening. 



2. The separation of the anal and branchial opening by a bridge 

 formed by the mantle. 



3. The contact of the anterior insertion of the inner gill with the 

 posterior insertion of the inner palpus. 



4. The absence of a posteriorly produced point of the palpi. 



5. The marsupium, which is restricted to the inner gills. 



6. The peculiar shape of the septa of the marsupial gill. 

 Among North American shells, I have observed a similar absence 



of the supraanal opening only in Lampsilis parva (Barn.), but this 

 is a secondary modification: is not always present, and all the related 

 forms have such an opening. In Margaritana and certain species of 

 Quadrula (^Rolundaria^ , where a supraanal is also absent, the mantle 

 edges have no tendency to close the anal above, and thus represent 

 another type of structure. With these exceptions, I have never seen 

 any one of the above characters in any species of North America. 

 There is always a distinct supraanal opening; the separation of the 

 anal and branchial openings is never formed by the mantle edge, and 

 the diaphragm consists of the gills alone; the anterior insertion of 

 the inner gill is always more or less, generally widely, separated 

 from the posterior insertion of the palpi; the palpi are always drawn 

 out into a posterior, projecting point, and are more or less falcate; 

 and the marsupium is never in the inner gill alone, but either in 

 both gills or in the outer one. As regards the shape of the septa of 

 the marsupial gill, it is unlike anything I have seen in North Ameri- 

 can forms. Unfortunately 1 cannot tell how far this shape changes 

 in the gravid females, but I tliink, that the swelling of the gill, when 

 charged, will not be very considerable. 



It is clear, that these are very important differences. Simpson 

 (Pr. U. S. Mus. 22, '00, pp. 515 and 895) places Spatha in the 

 family Mutelida, the chief characters of which are the hinge teeth, 

 which, when present, are taxodont, and the shape of the embryo, 



