NO. 1395. CAMBJU. I .V BRA CFTTOPODA— WALCOTT. 241 



the shell. The deltli\ liiiiii is well dctined mikI covered by a convex 

 deltidium, the extent of which, however, is unknown. The cardintd 

 area of the dorsal valve is short. No details of the structure have 

 been observed. 



The cast of the interior of the ventral valve indicates short hinge 

 teeth supported b}' dental plates. The markings left on the shell bj^ 

 the vascular system, as shown in casts, are two main vascular trunks 

 which extend nearly in a straight line from the side of the cast of the 

 deltidial cavity obliquely outward to the anterior fourth of the valve, 

 where they bifurcate. The inner branch extends in toward the median 

 line, while the lateral branches extend outward, appearing to unite 

 with the peripheral canal. The latter arches about the space that was 

 probably occupied by the ovarian areas, between itself and the main 

 vascular trunks. The spaces for the attachment of the muscles between 

 the main vascular trunks appear to have been large. No subdivision 

 indicating the points of attachment of the different muscles have been 

 detected. At the posterior portion of the central area there is a 

 narrow, elevated space that posteriorly passes into the deltidial cavity. 

 The pedicle muscles were probably attached to the front part of this 

 elevated space. Short, radiating striae near the margin indicate the 

 ])resence of minute vascular canals. 



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

 supports a small, well-developed cardinal process or callosity. The 

 crura are short and small, with rounded dental sockets beside them. 

 Nothing is definitely shown of the vascular trunks or mesial impres- 

 sions in the dorsal valve. 



Ohservationx. — This very pretty little species occurs in large num- 

 bers in the Gallatin limestone, Montana. In form it is not unlike the 

 convex forms that have been referred to B. coloradoensis from Trem- 

 pealeau, Wisconsin. It differs from the latter in having stronglj" 

 marked surface plications and a narrower deltidium. From B. striata 

 it differs in surface characters; also in having a more convex dorsal 

 valve. 



Formation and localities. — Upper Cambrian. Gallatin limestone, 

 east side of Dry Creek, above Pass Creek, Gallatin Valley; west side 

 of Dry Creek not far from mouth of Pass Creek; north of East Gal- 

 latin River near Hillsdale; on ridge between Churn and Cottonwood 

 canyons, west side of Bridger Range, Montana. 



All representatives of this species were collected by Dr. A. C. Peale, 

 of the Hayden survey, except some obscure specimens 1 found at the 

 base of the limestone above the Flathead shales G miles north of York, 

 in the Big Belt Mountains, Montana. 



