8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



in Camarophorina Licharev, Psilocamara Cooper, and normally absent 

 in Camerisma n. gen. 



On the specific level the strength and distribution of costae are 

 important. In Atribonium n. gen., their relative strength on fold or 

 flanks is significant. In Stenoscisma Conrad, their beginnings on the 

 umbones are important ; in Sedenticellula Cooper, their relative 

 strength and bifurcations distinguish species. In Coledium n. gen., the 

 relative strength of costae on the fold and flanks helps to determine 

 species. 



Evolution. — Costae show progressive increase in strength through 

 the development of the Stenoscismatidae, but they exhibit no definite 

 trend within the Atriboniidae. Costae are relatively weak and devel- 

 oped only anteriorly in the Devonian Atribonium, but they are nu- 

 merous and narrow in the Mississippian Sedenticellula. Costae begin 

 far anterior in the atriboniid Cyrolexis n. gen. which occurs primarily 

 in the Permian. Camarophorinella Licharev has moderately strong 

 costae ; others of the family are smooth or nearly smooth. 



Among the Stenoscismatidae, costae exhibit an erratic but persistent 

 trend toward increase in size and extent, from the Mississippian 

 through the Permian. The earliest known genus with a stolidium is 

 Coledium n. gen. (Mississippian) which has some smooth species, and 

 some whose costae commonly begin about midlength on the shell. 

 Costae in Stenoscisma Conrad increase in strength from the earliest 

 known species in the Lower Carboniferous of Britain, through Per- 

 mian species of Germany, the Urals, Timor, and North and South 

 America. The type species, .S. schlotheimi (von Buch), has individuals 

 with weak, nearly absent costae, and others with relatively strong 

 costae ; however, nearly all have costae beginning several millimeters 

 anterior to the beaks (pi. 21 ) . 



Species of Stenoscisma from the Permian of Timor (Broili, 1916) 

 have strong or weak costae ; species from Texas have costae ranging 

 from short and anterior, like those on 5. schlotheimi (e.g., 5\ thevenini 

 (Kozlowski) in King, 1931) to those that cover the entire shell, ex- 

 cepting only the extreme posterior, as in 5". venustum (Girty), 5". ka- 

 lum Stehli, and 6". multicostum Stehli. 



THE STOLIDIUM 

 Description. — The broad frill or skirt around the anterior and antero- 

 lateral commissures of genera of the Stenoscismatinae is here termed 

 the stolidium, from the Greek stolidion : a skirt. The stolidium is pres- 

 ent only on adult specimens, where its form varies greatly among the 

 species {see pis. 22, 23; fig. 2). It is narrow in species of Coledium 

 n. gen., but normally it is broad in species of Stenoscisma, reduced in 



