BILLINGSELLID.^. ' 755 



bound the spaces occupied by the point of attachment of the diductor muscles. Linnarsson 

 describes [1876, p. 12] the cardinal process as triangular. I find it both straight (fig. If), 

 subtriangular (fig. li), and triangular (figs. Ig and Ih). A short median septum is seen in 

 some shells (fig. Ig), but usually the strong, shghtly rounded, flattened median ridge does 

 not carry it. 



Ohservations. — This is one of the most variable species of the genus, both in external, 

 and, as far as known, internal characters. It is very abundant, but interiors of the ventral 

 valve are rarely met with in good condition. 



The extremely short, imperforate deltidium of the ventral valve indicates that the pedicle 

 opening had been closed by shell growth, and that this species was nearing the Eoorthis rem- 

 nicha stage of development. It appears to be an intermediate form between BUlingseUa and 

 Eoorthis. The concave deltidium is niucli like that of ConcMdium nysius Hall and Wliitfield 

 [Hall and Clarke, 1894, PI. LXIV, fig. 7]. It is about one-fifth the length of the delthyrium. 



Linnarsson [1876, p. 12] compares "Orthis exporr'ecta" with "OrtMs Mclcsi," wlrich appeared 

 to be most nearly related, pointing out that it differed in having fewer and coarser ribs, less 

 pointed beak, and lower area. The "Orthis hicksi" variety is now placed as a variety of 

 BiUingseUa exporrecta. 



Formation and locality. — Upper? Cambrian: (310h) Shale collected somewhere (probably Fogelsang) in the 

 old province of Skane, Sweden, 



Middle Cambrian: Limestones of the Paradoxides forchhammeri zone at the following localities: (320m) " at 

 Kinnekulle, northeast of Lidkoping, Province of Skaraborg; (320n) at Lovened, Djupadal, 19 miles (30.6 hm.) 

 south-southeast of Skara, Province of Skaraborg; (320y)a at Gudhem, 12.5 miles (20.1 km.) south-southeast of Skara, 

 Province of Skaraborg; (320p)a at Sodra Mockleby, southern part of Oeland Island; (320z)o several places in 

 Nerike; (330g) at Borgholm, Oeland Island; (321q) at Munkesten, north of Hunneberg; and (321r) at Hunneberg, 

 western boundary of the Province of Skaraborg; all in Sweden. 



(320 1)« Drift blocks supposed to have come from the Paradoxides cdandicus zone [Linnarsson, 1876, p. 6], at 

 Lillviken, near Oestersund, Province of Jemtland; (321p [Wallerius, 1895, p. 67]) in strata with Agnostus Isevigatus 

 at Carlfors, near the north end of Mount Billingen, Province of Wermland; (SlOz) at Brantevik, on the shore a little 

 south of Simrishamn, Province of Christianstad ; (320b) limestones forming 2d of th.e Paradoxides 7,oaQ,\h.e Agnostus 

 tew'sraiits horizon, at Gudhem, 12.5 miles (20.1 km.) south-southeast of Skara, Province of Skaraborg; and (310b) dark- 

 brown limestone at Borgholm, Oeland Island; all in Sweden. 



BiLLINGSELLA EXPORRECTA RUGOSICOSTATA Walcott. 

 Plate LXXXVIll, figures 2, 2a-c. 



Orthis hicksi Linnarsson [not Salter MS., Davidson], 1876, Bihang till K. svensk. Vet. -Acad. Handl., Bd, 3, No. 



12, pp. 13-14, PL III, figs. 22 and 23. (Described and discussed in English.) 

 BiUingseUa exporrecta rugosicostata Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 236. (Discussed as below as a 



new variety.) 



Linnarsson [1876, p. 13] recognized that this shell was not the same as his "OrtMs exporrecta" 

 and placed it provisionallj^ under Orthis hicksi, callmg attention to the fact that it was nearly 

 related to 0. exporrecta. With a fine series of specimens for comparison, I place the shell as a 

 variety of B. exporrecta. It is characterized by its strong, irregular costse. Linnarsson [1876, 

 p. 14] gives as the most important differences: "Ventral valve less convex; cast of diductor 

 muscle scars less distinct; dorsal valve more convex and muscle scars more strongly marked; 

 radiating ribs in the casts fewer, stronger, and more angular." AU of the differences exist' 

 when we compare the extreme forms, but with a good series of specimens most of them pass 

 gradually into forms that are typical of B. exporrecta. 



Formation and locality. — Middle Cambrian : Limestones of the Paradoxides forchhammeri zone at the following 

 localities [Linnarsson, 1876, p. 14]: (320m) At Kinnekulle, northeast of Lidkoping, Province of Skaraborg; (320n) at 

 Lovened, Djupadal, 19 miles (30.6 km.) south-southeast of Skara, Province of Skaraborg; and (330p) at Sodra Mock- 

 leby, southern part of Oeland Island; all in Sweden. 



(320b) Limestone forming 2d of the Paradoxides zone, the Agnostus Isevigatus horizon, at Gudhem, 12.5 miles (20.1 km.) 

 south-southeast of Skara, Province of Skaraborg, Sweden. 



a Linnarsson, 1876, p. 13. 



