42 THE NAUTILUS. 



(1900, p. 670) in the genus Alasmidonta. However, the chief 

 character of Alasmidonta is in the beak sculpture, which is 

 heavy and generally concentric, at the outmost with only a 

 slight indication of a sinus. And, further, some species of 

 Alasmidonta have a tendency to have the inner lamina of the 

 inner gill more or less connected with the abdominal sac (Ort- 

 mann, 1912, pp. 279, 280, 294). These are the only essential 

 characters which distinguish this genus from Symphynota 

 Lea. 



Marg. holstonia has a beak sculpture which is not heavy, 

 consisting of four to six rather fine and sharp bars, the first 

 one or two subconcentric, the following ones sharply double 

 looped, the posterior loop smaller, separated from the anterior 

 by a deep, sharp, re-entering angle. This sculpture is iden- 

 tical with that of Symphynota compressa, viridis, and com- 

 planata. Aud, further, M. holstonia has the inner lamina of 

 the inner gills free from the abdominal sac, agreeing also in 

 this with Symphynota. Thus it is evident, this species should 

 be placed here. 



Symphynota has three subgenera, according to Simpson, but 

 to none of these holstonia can be assigned, and thus it is best, 

 to create a new subgenus, Alasminota, for it. Its relation to 

 the other subgenera may be made clear by the following table : 



Genus Symphynota Lea. 



a 1 Hermaphroditic. Cardinal and lateral teeth present. Beak 

 sculpture sharply double-looped. Shell subovate or sub- 

 trapezoidal, moderately long. 



Subgenus: Symphynota Simpson. 



a 2 Gonochoristic. Cardinal teeth present, laterals rudiment- 

 ary or absent. 



b 1 Shell elongated elliptical, rather small. Surface with- 

 out sculpture. Beak sculpture sharply double-looped. 



Subgenus: Alasminota Ortmann. 



b 2 Shell subrhomboid, subtrapezoidal, or subovate, moder- 

 ately long or short, quite large. Surface with more 

 or less developed sculpture of radiating ridges upon 

 the posterior slope. 



