236 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



Doctor Linnarsson compares O. expovrecta with Orthis hicksi which 

 appeared to be the most nearly related, pointing out that it differed 

 in having fewer and coarser ribs, less pointed beak and lower area. 



Foriuation and locality . — Middle Caml)rian. Zone of Paradoxides 

 forch/iaiiinierl. I^innarsson's localities are Kinnehiille, Lovened and 

 Gudhem in Westrogothia; also at several places in Nesike; at Sodra 

 Mockleby in Oland; and in loose stones at Lillviken in Jemtland. In 

 strata with Agnostus laevigatus at Carlsfors in Westrogothia, Sweden. 



The specimens I have illustrated were collected by M. Schmalensee 

 in Westrogothia. 



BILLINGSELLA EXPORRECTA, var. RUGOSICOSTATA, new variety. 



Orthis Mcksi Linnarsson, Bihang till K. Svenska Yet. Acad. Handlingar, III, 

 1876, No 12; Brach. Paradoxides Beds of Sweden, p. 13, pi. in, figs. 22, 23. 



Doctor Linnarsson recognized that this shell was not the same as his 

 Orthis exporrecta and placed it provisionally under Orthis hicksi, call- 

 ing attention to the fact that it was nearlj^ related to 0. exporrecta. 

 With a tine series of specimens for comparison I place the shell as a 

 variet}^ of B. exporrecta. It is characterized by its strong irregular 

 cost*. Doctor Linnarsson indicated as the most important differences; 

 "ventral valve less convex; cast of diductor muscle scars less distinct; 

 dorsal valve more convex and muscle scars more strongh^ marked; 

 radiating ribs in the casts fewer, stronger, and more angular." All 

 of the differences exist when we compare the extreme forms, but with 

 a good series of specimens most of them pass graduall}^ into forms that 

 are tj'^pical of J^. exporrecta. 



Fonnatioii and locality. — Middle Cambrian. Zone of Paridoxides 

 fordihainmeri. Doctor Linnarsson found this shell at Kinnekulle and 

 Lovened in Westrogothia and at Sodra Mockleby in the island of 

 Oland, Sweden. The specimens I have illustrated were collected by 

 M. Schmalensee in Westrogothia. 



BILLINGSELLA HARLANENSIS, new species. 



The general form and convexity of this shell is similar to that of B. 

 plicatella. It differs in having a nearly smooth surface marked only 

 by fine concentric striae and a few traces of sharp radiating costse. 

 The interior of the ventral valve has very strong main vascular sin- 

 uses and a deep, sharply defined, tripartite umboral area opposite the 

 delthyrium; the tripartite area is much like that in B. coloradoensis 

 and B. exporrecta. The main vascular sinuses pass directly back across 

 the ridge in front of the tripartite area and occupy the two lateral 

 divisions of the area. The interior of a dorsal valve, shows an abnor- 

 mal arrangement of the vascular markings and muscle scars. 



Foniiatlo)) and locality. — Middle Cambrian. Limestone layers in 

 Kogersville shale, 4 miles northeast of Rogei'sville and 11 miles north- 

 west of Knoxville, Tennessee. 



