88 THE NAUTII,U8. 



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

 sory brealliing apparatus amonj^ the land <»perculates. is to be 

 questioned. The necessity for obtaining air when the aj)erture 

 is tightly sealed with the operculum has apparently caused 

 many genera not at all closely related to develop ingenious and 

 occasionally sonjewhat similar breathing contrivances. Species 

 of Pterocyclos, Spiraculum, Rhiostoma and Tomocyclos, while 

 not closely related to our American land operculates have de- 

 yeloped 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. 



Mrst. With perforation connecting with visible external tube. 



a. Operculum of Rhytidopoma. 



1. Tube greatly prolonged, entering umbilicus. 



Opisthosiphon rvg'domm Pfr. Matanzas. 

 Opisthosiphon denegntum Poey. Isle of Pines. 



2. Tube short, external opening towards and close to 



preceding whorl. 

 Opisthosiphov hahaviense Sh. Bahamas. 

 S. Tube short, disappearing in the suture between 

 the ultimate and penultimate whorl. 

 Oputhosiphon pvpnide* Mor. Isle of Pines. 

 4. Tube short, straiglit, not recurved. 



Opisthosiphon xculptum Gundl. Cabo Cruz. 



b. Operculum of Choanopoma. 



1. Tube as in Opisthosiphon pnpoides Mor. 



Choanopoma uncinntum Arango. Sta. Clara, 

 Cuba. 

 Second. With perforation opening directly into umbilical 

 region or exterior of shell. 



a. Operculum of Choanopoma. 



Choanopoma blaini Gundl. Galalon, Cuba. 



b. W'ith operculum of Cistula. 



Cvstula limbijera Mke. Matanzas. 



