No. 2327. THE ANNULARIIDAE— HENDERSON AND BABTSCH. 65 



outer surface of the lamellae bears a calcareous deposit which may- 

 consist of simple retractively curved riblets that may remain dis- 

 tinct or may become fused into a soHd plate. These deposits may 

 cover the entire width of the whorl or only a fraction thereof. The 

 inner and outer termination of the ribs ma}^ or may not fuse to form 

 a strong spiral lamella, but in no instance does the inner lamella 

 rise up as a simple spiral band as in the Adamsiellinae nor become 

 reflected outward and calcified as a second or roofing plate as in the 

 Annularinae. 



Type genus. — Rhytidopoma Sykes. 



RHYTIDOTHYRA, new genus. 



Shell elongate-ovate, marked by both axial and spiral threads. 

 The breathing pore communicates with a tube located in the posterior 

 angle, extending backward for more than three whorls. This tube, 

 in turn, communicates, by slender clefts, with the hollow axis, 

 through which breathing is evidently effected when the operculum 

 is closed. The operculum consists of many strongly raised, retrac- 

 tively curved lamellar ribs, which extend completely across the turns. 

 These lamellar ribs are high at their inner edge, from which they pass 

 down in a gentle curve, rising again to their highest altitude at their 

 outer border, where they become fused into a spiral lamella, which 

 projects almost as far as the basal chondroid plate. The lateral 

 margin of the operculum is strongly channeled. 



Type of the genus. — Cyclostoma Ulabiatum Orbigny. 



Genus PARACHONDRIA Dall. 



1905, Parachondria Dall, Proc. Malac. Soc. London, vol. 6, p. 209. 



Shell ranging in shape from elongate-ovate to elongate-conic, 

 marked by axial ribs only or b}^ axial ribs and spiral threads which 

 are confined to the umbilicus or by axial and spiral threads on spire 

 and base or slender axial lamellae and subobsolete spiral thi'eads 

 which give to the axial sculpture an articulate appearance. No 

 breathing device is present. The operculum has the inner portion 

 of its turns covered by a calcareous deposit which consists of numerous 

 low retractively curved fused riblets, which are not fused into a 

 raised lamella at their outer edge, nor does the calcareous deposit 

 extend to the edge of the chondroid basal plate. There is thus left a 

 narrow channel in the bottom of which the chrondroid plate may be 

 seen at the outer edge of each whorl. 



Type oftlie genus. — Turbo fascia Wood. 



PARACHONDRISCA, new subgenus. 

 Shell elongate-conic, marked by axial riblets only. No trace of 

 spiral riblets, even in the umbihcus, is present. Operculum typically 

 Parachondroid. 



181404— 21— Proc. N.M.vol.58 5 



