672 PROCEEDINGS OF THE NATIONAL MUSEUM. 



course of the parietal sear between the main vascular sinuses is in 

 front of the splanchnocwle in the Aentral valve; also in the same valves 

 it passes around the muscle scars between the main sinuses and the 

 base of the area, in line with the flexure line of the area; in the dorsal 

 valve it closely follows the outside limits of the muscle scars, but it 

 has not been traced across the main vascular sinuses. 



The size and position of the pediele and uml)onal muscle scars are 

 unknown, but they are probably similar to those of Obolus. The cen- 

 tral scars are placed a little distance each side of the median line in 

 the dorsal vahe. In the ventral valve they are not separable from 

 the middle and outside laterals, which occur on each side of the front 

 of the visceral area. The middle and outside laterals of the dorsal 

 valve are blended and lie obliquely outward, before the transmedian 

 scars. The transmedian scars are close to the base of the area in both 

 valves, and lie in the line of the prolongation of the flexure line of 

 the area. In number and relative position the muscle scars of 0]:)olena 

 are essentially the same as those of Obolus. 



Oh><ervatlons.—ThQ genus Obolella has been under discussion by 

 authors for nearly forty years. In the original description :Mr. Bil- 

 lings noted its resemblance to Obolus, but claimed that it is distinct 

 on account of the difference in "the arrangement of the muscular 

 impressions." Later (187(3) he compared it again with Obolus, but 

 having poor illustrations of both Obolus and Obolella ho failed to dis- 

 cover the true position and relations of the muscle scars in either. It 

 was not until after Mickwitz's memoir on Obolus appeared, and the 

 collections of Obolella made during the summer of 1899 were studied, 

 that any correct comparisons could be made. It then became evident 

 that there is practically no difference in the arrangement of the 

 muscle scars of the two genera, but that there are most essential dif- 

 ferences in the areas of the ventral valve. Before working out the 

 relations of the narrow pedicle slit of Obolella to the cylindro-conical 

 pedicle chamber, I was at a loss to find generic differences of value, 

 although I felt that the area of the dorsal valve of Obolella indicated 

 differences not readily explained. 



The pedicle passage of Obolus varies greatly in size and form, l)ut 

 it is always an open furrow. In Obolella it is in the ventral valve a 

 cylindro-conical inner chamber opening through a narrow slit in the 

 area of the ventral valve, the area rising from the plane of the margin 

 of the valve; in the ventral valve a slightly raised area occupies the 

 place of the broad furrow in Obolus. 



Obolella is confined to the Olenellus or Lower Cambrian fauna. 

 Obolus, as now limited, appears in the Middle Cambrian, Init has its 

 greatest development in the Upper Cambrian fauna. Of the subge- 

 nera Oholiis {Lingidelhi) schuchertl is found associated with Obolus 

 ( O. erassa) at Troy, New York. 

 Bkiagtinina is associated with Ololdla C7'<6y.s(c both at Bicand Trov 



