400 I'lKHJEIiDlNdS OF THE NATIONAL Ml'SKUM. vol. xxr. 



inner surface. Tlie >shell is tliin and lornie<l of an outer layer aud oue 

 or more inner layers or lamella'. 



The average length of the ventral valve is about 4 mm.; width, 3 

 mm. A dorsal valve .■>.5 mm. loug has a width of 'A mm. 



A cast of the interior of a ventral valve shows an area of medium 

 length, divided midway by a narrow, clearly defined i)edicle groove. 

 The area of the doisal valve is short. Nothing is known of the inte 

 rior of the ventral valve, but in a cast of a dorsal valve there are 

 traces of the main vascular sinuses, central median septum, and the 

 central muscle scars. 



Observations. — This species may be compared with the Middle Cam- 

 brian 0. {L.) ferrugineus of the Atlantic Basin fauna, and 0. (L.) similis 

 of the Black Hills, Upper Mississippi Valley, and Appalachian faunas. 

 Compared with the llocky Mountain species it is intermediate between 

 0, (jL.) manticiUus and 0. {L.) rotundatus. It may also be compared 

 with 0. (L.) granmllensis of the Olenellus fauna of eastern New York, 

 and O. {L.) iole of the Lower Ordovician fauna of Newfoundland. 



The specimens that occur at the same geological horizon in the 

 Eureka district, Nevada, are broader proportionately than the typical 

 specimens, and what appears to be the same, or a closely related 

 species, occurs in the upi)er beds of the Secret Canyon shale just 

 beneath the Hamburg limestone, 1,200 feet lower in the Eureka Cam 

 brian section. 



A form that appears to be identical occurs in considerable abun- 

 dance in the shaly limestone at the base of the lower Knox of Ala- 

 bama, and also in similar limestones in Tennessee. This similarity is 

 even more striking when the specimens are compared directly with 

 each other and some allowance made for the fact that the Appalachian 

 specimens have all been more or less com])ressed. 



The Tennessee specimens occur in limestones interbedded in or at 

 the summit of the Upper Cambrian Connasauga shale, a short distance 

 beneath the Knox dolomite. Those from Alabama occur at the same 

 horizon on the south side of the Coosa Valley. Compressed specimens 

 referred to 0. {L.) similis are found in the Coosa shales at Yanceys 

 bend, Coosa liiver, Cherokee County, Alabama, that can scarcely, be 

 distinguished from (). (L.) desideratiis. 



A shell that api)ears to be identical with this species occurs in the 

 red sandstone and agrillaceous shale of the Lower Ordovician of Colo- 

 rado. Tlu^ specimens from Trout Creek below I>ergen Park are much 

 like those from the (iallatin Range, and the same species of Billing- 

 sella is associated with them. At Cement Creek, 10 miles southeast of 

 Crested Butte, the shells occur in a fine conglomerate and coarse sand- 

 stone, associated with a species of liatltyHrus much like that from the 

 beds containing 0. {L.) desideratus at Trout Creek. 



The vertical range of the shells referred to this si)ecies appears to be 

 from the upper beds of the Cambrian into the lower beds of the Ordo- 



