No. 2327. THE ANNULARIIDAE—HENDERl^ON AND BARTSCH. 51 



further and instead of a mere simple more or less erect spiral lamella, 

 we have the calcified lamella variously strengthened by calcified rib- 

 lets, or in extreme cases connected by trabecuiae with the calcareous 

 deposit on the basal plate. This is particularly true in such forms as 

 have the lamella reflected outward to parallel the basal plate. In 

 all the Annularmae the operculum has a double appearance, the outer 

 and mncr plate being separated by a concave groove at the free 

 border. In some Annularinid mollusks the lamellae completely cover 

 the space between succeedmg turns, and did one not remove the oper- 

 culum from the shell one would be left in doubt as to whether the 

 operculum was simple or double. 



The amount of calcification and the manner in which this takes 

 place is also interesting and points out anotlier line of progressive 

 modification. In the Chondropominae we may find a few scattered 

 calcareous granules, or enough of tliese to form a material thicken- 

 ing, but no matter what the amount, it is never built into ribs or 

 lamellae. In the Rhytidopominae, on the other hand, we find in the 

 simplest forms slender retractively placed riblets which radiate from 

 the inner edge of the whorls outward across either a part or the 

 whole whorl. In other members of the subfamily, these weak rib- 

 lets are replaced by strong ribs which may fuse to form a spiral 

 lamella at their inner, or outer, or both borders, but in no instance 

 have we found the inner lamellae thus formed arch outwardly to form 

 a plate, as is the case in the Annularinae, though in the Rhytidopomid 

 genus Xenopoma, the outer lamella is extraordinarily developed 

 and arches inwardly over the preceding turns, completely covering 

 them. 



BREATHING DEVICES. 



The breathing devices found in Annulariidae are very ingenious. 

 They range from a mere notch in the peripheral callus near the 

 posterior angle of the aperture, which leaves a slender opening when 

 the operculum is withdrawn, to a puncture in the parietal wall, 

 which may or may not be provided with a projecting tube (siphon) 

 on the outside. Some have a puncture in the parietal wall con- 

 nected with the outer surface of the peristome by a slit. In some 

 the puncture connects with an air chamber which passes back for 

 several turns in the parietal wall and which is in communication 

 with the hollow axis by a series of minute punctures. In some 

 forms the puncture communicates directly with the hollow axis of 

 the shell. The umbilicus being sealed by the parietal callus, the 

 animal breathes through the perforation at the truncated apex. 

 This apex breathing when the operculum is closed reaches its highest 

 development in Rhythidopoma, in which the siphon bends down 

 through the solute portion of the last turn over the base of t]ie 



