NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 233 



quite large, extending forward to the anterior fifth of the length of the 

 valve, but no subdivisions indicating the points of attachment of the dif- 

 ferent muscles have been detected. The pedicle nuiscles were prob- 

 jibly 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 quite prominent. 



In the interior of the dorsal valve the interior of the deltidial cavity 

 supports a small, well-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 dehned, 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 l-I mm. in height for the ventral valve, the width being 

 about the same. At one locality on Morgan's Creek several dorsal 

 valves were found that have a width of 18 mm., with a height of 

 12 mm. This ma}" possibly indicate a variety or distinct species, but 

 with the material in the collection it is impossible to determine it 

 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 Range, Yellowstone National Park, is also much like that 

 from Texas, but that from the limestone near Malad City, Idaho, 

 while containing typical shells, also has specimens as large as those 

 from Morgan's Creek, Texas. 



BiUlngsella major differs from B. coloradoensis in the character of 

 the surface striation, also in its larger size. B. plicatella is a unformly 

 smaller and more convex shell, and also has distinct surface characters. 

 The same is true of B. striata. 



This species has a wide geographic range that extends from Texas 

 to Missouri, Minnesota, and Wisconsin, and westward into Montana, 

 Wyoming, and Idaho. Its vertical range in Texas is limited to the 

 upper portion of the Middle Cambrian, and in Wisconsin and Minne- 

 sota to the St. Croix sandstone from the upper portion of the zone of 

 the Middle Cambrian horizon nearly through the zone of the Upper 

 Cambrian fauna; in Idaho to the Middle Cambrian; in the upper Gal- 

 latin Valley of Wyoming to the Upper Cambrian; and probably the 

 same horizon in the Gallatin Valley below in Montana. There does 

 not appear to be any specific variation in the shells from Texas, the 

 upper Mississippi Valley, and the Upper Cambrian of Wyoming. 



FoTination and locality. — Middle and Upper Cambrian. 



