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University of California Publications in Geology [Vol. 13 



This fossil has hitherto been regarded as belonging to the genus 

 Agasoma Gabb. 34 English 35 in his description of the genus Agasoma 

 stated that there were two sections of that genus, but he did not give 

 them separate names. He stated : ' ' The first section includes only 

 A. sinuatum Gabb, with the narrow mouth opening; narrow, deep, 

 medium length, recurved canal ; and the pronounced angulation of the 

 body whorl. The second includes the other species with evenly 

 rounded, ventricose body whorl, and shallow wide canal." This 

 second section is the typical Agasoma described by Gabb. 



In view of the spine on the outer lip ; depression on lower part of 

 body whorl ; pronounced subperf orate umbilicus ; and canal similar to 

 Acanthina Fischer de Waldheim, 36 it appears that this form shows a 

 closer relationship to Acanthina than to Agasoma. It differs from 

 Acanthina, however, in possessing a deeply channeled suture, a pro- 

 nounced collar; and two angulations on the body whorl with the 

 pronounced concavity between. Hence, due to these differences from 

 Acanthina it is regarded as a new genus. Since it shows a closer 

 relationship to Acanthina than to any other genus it is placed in the 

 Thaisidae family. 



The word "Koilopleura" is derived from /co/Xo?, concave, irXevpd, 

 side. 



Genus KOILOPLEURA, n. gen. 

 KOILOPLEURA SINUATA (Gabb) 



Clavella sinuatum. Gabb, " Calif. State Geol. Surv., Paleontology of Cali- 

 fornia, vol. 2, p. 5, 1869. 



Agasoma sinuatum. Gabb, Calif. State Geol. Surv., Paleontology of Calif., 

 vol. 2, p. 46, pi. 1, fig. 7, 1869. 



Agasoma sinuatum Gabb. English, Univ. Calif. Publ. Bull. Dept. Geol., 

 vol 8, p. 250, pi. 25, figs. 5 and 6, 1914. 

 Type. — No. 11994; cotype— No. 11995, Univ. Calif. Mus. Pal. 



An examination of Gabb's types, which are quite small specimens, 

 shows that what Gabb has taken for the convex portion in the middle 

 of the body whorl is the rounded lower angulation which becomes so 

 prominent in the older specimens. Between the two angulations there 

 is the same pronounced concavity seen in the larger specimens. 



3* Gabb, Pal. of Calif., vol. 2, p. 46, 1869. 



35 English, W. A., The Agasoma-like Gastropods of the California Tertiary, 

 Univ. Calif. Publ. Bull. Dept. Geol., vol. 8, p. 245, 1914. 

 se Fischer de Waldheim, Mus. Demid, 1806. 



37 Shell elongated, rather slender; spire low, convex; whorls four; suture 

 deeply channeled, bordered by a thickened rim; body whorl convex in the 

 middle, broadly grooved above, and excavated below; aperture long and narrow; 

 columbella sinuous, slightly incrusted; outer lip simple; canal slightly recurved. 



