THE NAUTILUS. 59 



forms which belong here. According to obesity, I distinguish 

 three local, or ecological races: 



1. FuscoNAiA BARNEsiANA (Lea) 1838. 



U. barnesianus Lea, '38. U. m&redithi Lea, '58. U. pudicus 

 Lea, '60. U. Lyoni Lea, '65, U. tellicoensis Lea, '72. U. 

 lenticularis Lea, '72. 



As the normal (most abundant) forms we may regard U. mere- 

 dithi, pudicus and lenticularis, which differ from each other only 

 in the development of the rays (topotypes examined). U. 

 barnesianus is a slightly more elongated individual, with poorly 

 developed rays. U. tellicoensis (topotypes examined) is a lenti- 

 cularis slightly more swollen; and U. lyoni forms the transition 

 toward var. tumescens, having a little more elevated beaks, greater 

 obesity, and rather distinct rays. 



2. FUSCONAIA BARNESIANA BIGBYENSIS (Lea) 1841. 



U. bigbyensis Lea, '41. U. estabrookianus Lea, '45. U. fas- 

 sinans Lea, '68. Pleurobema fassinans rhomboidea Simpson, '00. 



The most frequent form is fassinans rhomboidea (topotypes 

 examined), with rays poorly developed. U. bigbyensis has 

 more distinct rays; U. estabrookianus (topotypes examined) is 

 an old, overgrown form, without rays; U. fassinans is founded 

 upon an individual (type examined, also topotypes), which is 

 exceptionally elongated, without rays. 



3. FUSCONAIA BARNESIANA TUMESCENS (Lea) 1845. 



U. tumescens Lea, '45. U. crudus Lea, '71. U. radiosus 

 Lea, '71. 



U. tumescens is the most typical form, greatly swollen, with 

 more or less developed rays; U. radiosus (type and topotypes 

 examined) is less swollen, but for the rest like tumescens; U. 

 crudus (topotypes examined) lacks rays, and has much eroded 

 beaks, but stands close to radiosus. 



The mutual relations of these forms may be understood by 

 the help of the following key. Only the three largest divisions 

 are to be regarded as varieties, in the other forms the characters 

 are merely individual, although specimens representing only 



