244 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



iiio- with the ])oriphcral canal tliat aiches ])ackvvar(l jiliout the ovarian 

 areas between it and the main vascular trunks. No points of attach- 

 ment of the various muscles in the ventral valve have been observed. 

 Th(», area in whicih the diductor muscles are attached is well marked in 

 some specimens, and the cardinal process of the dorsal valve in others. 

 There are also shown in the dorsal valve the anterior adductor impres- 

 sions. In the interior of the dorsal valve the deltidial cavity separates 

 a small cardinal process or callosity, which is straight or subtriangular. 

 The crura are relatively" long- and \'ery pi'ominent, with distinctly 

 defined dental sockets beside them. 



Oh^iervations. — This species was well illustrated by M. Barrand. 

 In the material of the collections of the Museum of Comparative 

 Zoology, Cambridge, Massachusetts, there is a fine series illustrating- 

 the interior of the ventral and dorsal valves, that through the kindness 

 of Dr. Alexander Agassiz I was enabled to study. 



The species in its external and internal shape varies so decidedly 

 from all described forms that it is not necessary to point out difi'er- 

 ences between them. 



Formation and locality. — Middle Cambrian. Argillites and fine- 

 grained sandstone near Skrej, Bohemia. 



BILLINGSELLA SAFFORDI, new species. 



General outline of ventral valve subsemicircular and of dorsal valve 

 transversely broad oval; greatest width of valves at about the center; 

 hinge line straight and a little shorter than the greatest width of the 

 shell. A typical ventral valve has a length of 9 mm., width 9 mm. 

 Dorsal valve, length 10 mm., Avidth 12 mm. Biconvex, the ventral 

 valve being- more elevated at the umbo than the dorsal. Surface as 

 far as known smooth or marked bj" concentric strise and lines of 

 growth. 



Ventral valve with umbo curving- over and terminating in the small 

 apex that incurves a little over the area; area about one-half the ele- 

 vation of the valve, a broad delthyrium is partially covered by a 

 convex deltidium; casts of the interior indicate a low, tripartite 

 pseudospondylium, and one cast shows strong main vascular sinuses 

 extending from the lateral divisions of the pseudospondylium nearly 

 to the front margin. Dorsal valve with low area and small pseudo- 

 cruralium; th(> casts are too imperfect to show any other details. 



Ohservatvmx. — This species occurs in the same region as B. liarlan- 

 ensin and also has a smooth shell; it differs in being of equal length 

 and ])readtli and in having- less strongly marked interior characters. 



The species is named after Prof. James M. Safi'ord, former State 

 geologist of Tennessee. 



Foi'iitutioii andlocality. — Middle Cambrian. Rome sandstone, along 

 First Creek Cap, 4 miles north-northc^ast of Knoxville, Tennessee. 



