NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 235 



Shell transversely siibsemicircular; with the hinge line usuali}^ a 

 little shorter than the greatest width. In a few examples it is a trifle 

 longer; the cardinal angles are obtusely angular. The proportions 

 between the length and breadth vary. The ventral valve is consider- 

 ably more convex than the dorsal. It is evenly rounded while the dorsal 

 \'a\\q usually has a flattening of the median area, and sometimes a slight, 

 l)road sinus. The surface is marked by numerous radiating costiv and 

 very flne concentric striae between the costa% and a few visible lines of 

 growth. The cost* are rounded, usually, but not always, broader than 

 the interspaces and continuous from the umbo to the front and lateral 

 margins; they appear to be quite regular, although varying much in 

 size, and increasing somewhat irregularly by interpolation of new 

 costge; this usually occurs near the umbo, but may occur near the middle 

 of the valve. The largest ventral valve in the collection has a length 

 of 9 mm. and breadth of 11 mm. Dorsal valve, length 7 mm., breadth 

 10 mm. Cardinal area of ventral valve high and overhanging the 

 posterior margin. The delthyrium is large and appears to have just 

 a trace of a deltidium about its sides. Cardinal area of dorsal valve 

 short and slightly inclined forward; it is divided midway by a broad 

 delthyrium, which is divided by a strong cardinal process. 



In the interior of the ventral valve, strong, main, vascular sinu.ses 

 extend well forward toward the front margin; the cast of the umbonal 

 cavit}^ opposite the delthyrium shows a tripartite division, the center of 

 which is the area of the path of advance of the diductor muscle impres- 

 sions, and those on each side appear to be the posterior portion of the 

 main vascular sinuses. The anterior edge of the cavity is marked by a 

 sharp, elevated, forward arching ridge that extends to the teeth on either 

 side; the dental plates appear to have extended to the bottom of the 

 valve, and to have bounded the sides of the triangular umbonal cavity 

 opposite the delthyrium. 



The interiors of the dorsal valves are beautifully preserved. The 

 cardinal process, crura, crural plates, area, anterior and posterior 

 adductor nmscle scars and vascular sinuses are very distinct; the dental 

 sockets are shallow, but clearly shown. In one specimen there appear 

 to be two lateral supporting septa to the thickening before the crural 

 plates that bound the spaces occupied by the point of attachment of 

 the diductor muscles. Doctor Linnarsson describes the cardinal proc- 

 ess as triangular. 1 find it both straight, subtriangular, and triangular. 

 A short median septum is shown in some shells, but usually the strong, 

 slightly rounded, flattened, median ridge does not carry it. 



Ohse)"vatio)is. — 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. 



