NO.iir-2. CAMIiUIAN I'.RACHIOFODA—WALCOTT. 409 



doisiil viilvo is slightly sliortcr in proportion to the width. The average 

 size is smaller, not exceeding 5 mm. for the length of the ventral valve. 

 One unusually large ventral valve that is referred to this si)ecies with 

 some doubt lias a length of 9 mm. 



The only traces of the interior of the shell that have been observed 

 are portions of the area and pedicle farrow of the ventral valve and 

 the area of a dorsal valve. 



OhHercaUons. — This species is very abundant in the uiiper beds of 

 the Tonto sandstone. It is associated with 0. (L.) euglyphus, and it is 

 often difficult when the two are in the form of imperfe(!t casts to dis- 

 tinguish l)etween the larger specimens of the two species. They are 

 readily distinguished when the shells are well preserved by the differ- 

 ence in surface markings and tlie more acuminate ventral valves of O. 

 (Jj.) linenlatus. The latter character, however, is not always of service, 

 especially in the larger shells. In form the ventral valve of this species 

 may be compared with O. (L.) ((cutangulus. 



Formation and locality. — Middle Cambrian, Tonto sandstone at the 

 head of Clinar, Kwagunt, and Nunkoweap canyons, Grand Canyon of 

 the Colorado, Arizona. 



Types.— Nos. 27324-0, U.S.N.M. 



OBOLUS (LINGULELLA) MOSIA var. OSCEOLA, new variety. 



There is considerable variation in form of O. {L.) mosia as it occurs 

 in the brown sandstone at Osceola Mills, and for the narrow, more elon- 

 gate variety the name osceola is proposed. It is an intermediate form 

 between 0. [L.) mosia and 0. (X.) pcrattenuatus. 



Formation and locality. — Upper Cambrian, St. Croix sandstone, 

 Osceola Mills, Wisconsin. 



Typc.—^o. 27327, U.S.N.M. 



OBOLUS (LINGULELLA) NANNO, new species. 



Shell very small; general form elongate ovate, with the ventral valve 

 subacuminate to acuminate and the dorsal valve elongate ovate in out- 

 line. The convexity of the two valves is moderate in the very small 

 shells, increasing slightly with the increase in si/e. Average length 

 of the ventral valve is about 2 mm. and that of the dorsal valve a little 

 less. 



The surface of the shell as it appears in the hard, finegrained, drab- 

 colored limestone is marked by fine, concentric stria- and very faint 

 traces of radiating stria'. 



Observations. — This minute species occurs in thin layers of limestone 

 interbedded in the Coosa shales. Its small size and acuminate ventral 

 valve distinguish it from other species. It is associated with Acrotreta 

 and fragments of trilobites. 



Formation and locality. — Middle Cambrian, limestones in Coosa 

 shales, Bluntsville Valley, Alabama. 



Ty2)e.—^o. 27328, U.S.N.M. 



