752 CAMBKIAN BKACHIOPODA. 



Cardinal area of ventral valve moderately high. It is marked by transverse striae of 

 growth that cross it parallel to its base and arch over the convex deltidium. The plane of the 

 area extends backward at an angle of about 10° to the plane of the margin of the shell. The 

 delthyrium is strong and covered by a convex deltidium, the front margin of which arches 

 back about one-fifth the length of the delthyrium. The deltidium is marked by concentric 

 strise of growth and fine radiating lines and a minute perforation near its apex. The cardinal 

 area of the dorsal valve is short as compared with the ventral valve. It extends backward 

 at an angle of about 45° to the plane of the margin of the valve. It is divided midway by a 

 strong delthyrium which is covered for about half its distance by a convex chilidium. 



In the interior of the ventral valve the hinge teeth appear to be supported by dental 

 plates that extend down to the bottom of the valve and bound the tripartite umbonal space 

 opposite the delthyrium into which the vascular sinuses extend on each side of the diductor 

 muscle impressions, or their path of advance. The traces of the vascular system are confined 

 to the main vascular trunks which extend forward nearly to the front margin, where in some 

 examples they are bifurcated. The inner branch extends in toward the median line, disap- 

 pearing in the numerous radiating depressions near the margin. The lateral branches appear 

 to connect with the peripheral canal that arches about the space probably occupied by the 

 ovarian areas, between itself and the main vascular trunks. The space for the attachment of 

 the muscles between the main vascular trunks appears to have been quite large, extending for- 

 ward to the anterior fifth of the length of the valve, but no subdivisions indicating the points 

 of attachment of the different muscles have been detected. The pedicle muscles were probably 

 attached to the elevated, posterior portion of this central area. In some casts this posterior 

 area is scarcely elevated above the plane of the interior. In others it is fairty prominent. 



In the interior of the dorsal valve the interior of the deltidial cavity supports a small w^U- 

 developed cardinal process or callosity, and a slight narrow median ridge occurs just in advance 

 of the deltidial cavity. The crura are short and well defined, with relatively shallow dental 

 sockets beside them. The cardinal process and crura vary in size and length in shells from 

 the same locality. The only traces of the muscle scars observed show the anterior adductor 

 impressions. The vascular trunks of the dorsal valve diverge from the central line about the 

 center of the shell, after passing around the adductor muscle impressions. 



Observations. — The average size of the typical specimens from Texas is from 10 to 14 mm. 

 in length for the ventral valve, the width being about the same. At one locality on Morgans 

 Creek several dorsal valves were found that have a width of 18 mm., with a length of 12 mm. 

 This may possibly indicate a variety or distinct species, but with the material in the collec- 

 tion it is impossible to determine definitely. The shells from the upper Mississippi Valley in 

 Wisconsin and Minnesota average about the same size as the typical forms from Texas. The 

 young shells are much more convex. The material from the Gallatin Eange, Yellowstone 

 National Park, is also much like that from Texas, but that from the limestone near Malade, 

 Idaho, though it contains typical shells, also has specimens as large as those from Morgans 

 Creek, Texas. 



BiUingseUa major differs from B. Colorado ensis in the character of the surface striation, 

 also in its larger size. BiUingseUa plicateUa is a uniformly smaller and more convex shell, and 

 also has distinct surface characters. The same is true of B. striata. 



Formation and locality. — Lower Ordovieian: (339 [Sardeson, 1896, pp. 95 and 96]) Oneota dolomite, Still- 

 water, Washington County, Minnesota. 



Upper Cambrian : (302ii) About 1,000 feet (305 m.) above the quartzitic sandstones in the ' ' Yogo limestone (Devono- 

 Silurian)" of W. H. Emmons [1907, p 34], on Rock Creek, Phillipsburg quadrangle (U. S. Geol. Survey), Granite 

 County; (302r) same horizon as Locality 302q, near Princeton, Phillipsburg quadrangle (U. S. Geol. Survey), Granite 

 County; (302f) limestone in upper part of the Gallatin formation, north side of Elk Pass, between Buffalo and Slough 

 Creeks, Yellowstone National Park, Livingston quadrangle (U. S. Geol. Survey); (147a) limestone in Spring Hill 

 Canyon, west side of the Bridger Range, Gallatin County; and (155) limestone north of East Gallatin River, near 

 Hillsdale, Threeforks quadrangle (U. S. Geol. Survey), Gallatin County; all in Montana. 



(302p) Middle limestone of the Deadwood formation near the summit of the Owl Creek Mountains, 18 miles (29 

 km.) southwest of Thermopolis, Fremont County; (302g) limestone on the north slope of Crowfoot Ridge south of the 



