88 THE NAUTILUS. 



yet it is true that the value for showing relationships, of acces- 

 sory breathing apparatus among the land operculates, is to be 

 questioned. The necessity for obtaining air when the aperture 

 is tightly sealed with the operculum has apparently caused 

 many genera not at all closely related to develop ingenious and 

 occasionally somewhat similar breathing contrivances. Species 

 of Pterocyclos, Spiraculum, Rhiostoma and Tomocyclos, while 

 not closely related to our American land operculates have de- 

 veloped breathing apparatus similar to that of some of the 

 American species. The American shells belonging to the 

 Ericiidae, the genera of which are founded to a great extent on 

 the characters of the operculum, show great variation in the 

 apparatus through which air is introduced into the lumen of 

 the whorl when the aperture is closed by the operculum. At 

 least three types of accessory breathing apparatus may be seen. 



First. With perforation connecting with visible external tube. 

 a. Operculum of Rhytidopoma. 



1. Tube greatly prolonged, entering umbilicus. 



Opisthosiphon rugvloaum Pfr. Matanzas. 

 Opisthosiphon deneyntum Poey. Isle of Pines. 



2. Tube short, external opening towards and close to 



preceding whorl. 



Opisthoviphon bahamese Sh. Bahamas. 

 S. Tube short, disappearing in the suture between 



the ultimate and penultimate whorl. 

 Opisthosiphon pupoides Mor. Isle of Pines. 

 4. Tube short, straight, not recurved. 



Opisthosiphon scidptum Gundl. Cabo Cruz. 

 6. Operculum of Choanopoma. 



1. Tube as in Opisthosiphon pupoides Mor. 



Choanopoma uncinatum Arango. Sta. Clara, 



Cuba. 

 Second. With perforation opening directly into umbilical 



region or exterior of shell. 

 a. Operculum of Choanopoma. 



Choanopoma hlaini Gundl. Galalon, Cuba. 

 6. W 7 ith operculum of Cistula. 



Cistula limbijera Mke. Matanzas. 



