N0.1229. CAMBRTAN BRACHIOrnDA—WAIJOTr. 679 



T imi). AVidth, averag-e, 4 mm.; largest, 5 mm. The dorsal valve is 

 about one-iifth shorter than the ventral. 



The area of the ventral valve is usually on the plane of the margins 

 of the valve, but many specimens show it rising ut angles varying 

 from I'-' to 10-^. It is high and narrow, somewhat as in Obolus {Lingu- 

 MJ(i) acarninatus Mickwitz, and divided midway 1)y a strong, rounded, 

 narrow pedielo furrow; the stri» of growth are rather coarse and arch 

 forward at the center and across the pedicle furrow, following the 

 contour of the base of the area; the position of the flexure line is some- 

 times clearly shown b}' a narrow depression. The area of the dorsal 

 valve is short and easily escapes observation except in well-preserved 

 shells. It sometimes has a slight central pedicle depression, and often 

 is onW a short, almost smooth surface extending well out on the cardinal 

 slopes of the valve. 



The muscle scars, as far as determined, are arranged as in Obolus. 

 The umbonal and pedicle scars have not been observed. The elongate 

 oval central scars are rather large in the dorsal valve, and situated on 

 each side of the strong- median ridg-e about the middle of the valve; 

 in the ventral valve they are crowded in with the scars of the middle 

 and outside laterals; the anterior laterals are barely discernible in one 

 specimen of the dorsal valve as small oval dots on the central ridge a 

 short distance in advance of the centrals. In the ventral valve they 

 are close to the base of the area and near the outer edge of the shell. 

 The middle and outside laterals in the ventral valve are situated in the 

 trapezoidal area, but do not appear to be separable on the specimens 

 in the collection. In the dorsal valve they are well shown in advance 

 of the transmedian scar. The latter in the ventral valve is merg^ed 

 with the anterior lateral. 



Of the vascular markings the main sinuses of the ventral valve are 

 about all that are clearly shown, although the position of the parietal 

 scar is indicated in advance of the center of the shell. One of the 

 most strongh^ marked characters of the dorsal valve is the median 

 ridge; it varies in strength and outline in different shells, but is usually 

 a prominent feature; it extends to the frontal margin in most shells, 

 but in some it narrows and is less prominent anteriorly. A few 

 specimens show a slight depression crossing it just in front of the 

 central scars, and one has two minute anterior lateral muscle scars 

 directly on the ridge, the parietal scar passing across just in front of 

 them. The thickened shell beneath the visceral cavitj" of the ventral 

 valve is present in nearly all adult shells; it varies greatly in size, 

 form, and thickness; in some valves it covers the entire area within 

 the parietal scar, and in others only a portion. The thickening in the 

 dorsal valve is along the posterior border of the central cavity; this is 

 best shown in the cast; the median ridge is also frequently mor(^ or 

 less enlaro-ed. 



