Ilalcott Jfunb 



FURTHER NEW CAMBRIAN BELLEROPHONT 



GASTROPODS 



By J. BROOKES KNIGHT 

 Research Associate in Paleontology, U. S. National Museum 



(With One Plate) 



As the result of efforts to probe as deeply as possible into the 

 origins of prosobranch gastropods I published some months ago a 

 paper describing four genera of Cambrian bellerophonts (Kjiight, 

 1947). Since that time I have met with specimens of two more 

 Cambrian species of bellerophonts, each seemingly representing still 

 another genus heretofore unknown. 



Both of these species and genera are of great interest. One, 

 Strepsodiscus major, n. gen., n. sp., appears to be the earliest bellero- 

 phont yet known except the questionable bellerophont Coreospira 

 Saito. Its discuslike form and deep V-shaped sinus without a slit 

 reminds one somewhat of the Ordovician genus Cyrtodisctts Perner 

 but the final half-whorl is out of contact with the others and the coil 

 is slightly asymmetrical. Oriented as a bellerophont, the right side 

 of some specimens protrudes slightly; oriented with this right side 

 as "upward," the coiling would be called sinistral, a feature that 

 may have interesting implications that will be discussed elsewhere. 



Chalaro strep sis praeciirsor, n. gen., n. sp., resembles Strepsodiscus 

 major superficially in its discuslike shape and its free final half-whorl, 

 but it is entirely symmetrical and has a deep slit, the earliest known 

 appearance of a slit on any gastropod. It resembles even more 

 significantly Tropidodiscus Meek and Worthen (Ordovician — Devo- 

 nian) since both have the discuslike shape and the deep slit, but 

 in Tropidodiscus the shell is closely coiled and even involute. I 

 regard Chalarostrepsis as probably the immediate forerunner of 

 Tropidodiscus. 



With these two, we now know six genera of quite certain Cambrian 

 bellerophonts and two more that are probably primitive bellerophonts, 

 Coreospira occurs just above or below the boundary of the Lower 

 and Middle Cambrian. Strepsodiscus, new, and Cycloholcus Knight 

 seem to be of Dresbachian age. Sinuella Knight appears to be of 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. Ill, NO. 3 



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