SIPHONOTRETIDiE. 611 



studying some material representing the species in the collections of the United States National 

 Museum I noticed the edge of a shell that, on working it free from the matrix, proved to be 

 a ventral valve; two dorsal valves occur on the same bit of rock. Subsequently Dr. J. F. 

 Whiteaves sent me a tablet from the collections of the Geological Survey of Canada, on which 

 six specimens were labeled "Obolella circe." No. 1 is a cast of the interior of the dorsal valve 

 of Obolella crassa (Hall), and No. 2 the exterior of the same species. Nos. 3 and 6 are ventral 

 valves of Quehecia circe, and Nos. 4 and 5 dorsal valves. 



With the ventral valve identified, it appeared that the shell was not an Obolella, but the 

 type of an undescribed genus in which the ventral valve is elevated at the apex, with strong 

 cardinal tubercles for the attachment of the posterior muscle scars, very much as in Acro- 

 treta, and Quehecia was proposed. The cardinal tubercles and the presence of what appears to 

 be the base of the cast of a foraminal tube suggest that the ventral valve was perforate. 

 Obolella, Quehecia, and Yorkia are all strong, calcareous, perforate shells of the older Cambrian 

 fauna, and are evidently closely related. 



Formation and locality. — Lower Cambrian: (3191 [Billings, 1872a, p. 220])°' Limestone bowlders in conglom- 

 erate at Trois Pistoles, on St. Lawrence River, below Quebec, Canada. 



(392c [Billings, 1872a, p. 218]) at the Straits of Belleisle, Labrador. 



FamUy SIPHONOTRETID^ Kutorga. 



Genus YOBKIA Walcott. 



Yorkia Walcott, 1897, Proc. U. S. Nat. Mus., vol. 19, p. 714. (Described and discussed as below as a new genus.) 

 Yorhia Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 146. (Classification of genus.) 



Shell inarticulate, subcircular to suborbicular in outUne. Apex of ventral valve perforate, 

 marginal, with a false area and apseudodeltidium. The cast of the interior of the valve shows 

 a foramen which penetrated obliqueh' upward and backward through the thick umbonal por- 

 tion of the shell. Two narrow furrows converge from the side of the pedicle opening toward 

 the longitudinal mesial depression and then diverge toward the anterolateral portions of the 

 shell. 



The dorsal valve has a well-defined area and an obscure pseudodeltidium. The interior 

 of this valve has a pair of broad, diverging, shear-shaped furrows passing directly forward 

 from the beak for a short distance and then diverging to the outer margin, but not sufficiently 

 to affect the outward curve of the inner margin. 



Shell substance probably calcareous. External surface marked with more or less promi- 

 nent concentric striae and lines of growth. 



Type. — Yorkia wanneri Walcott. 



Observations. — The area and pseudodeltidium of the ventral valve resemble the same 

 parts in Trematobolus insignis Matthew, but the pedicle opening is quite different, and gen- 

 erally the interior of the dorsal valves also serves to distinguish the forms. To Discinopsis 

 Matthew there is a resemblance in the markings of the interior of the dorsal valve. Disci- 

 nopsis, however, is founded upon a small, apparently corneous shell, which, so far as known, 

 has no area or pseudodeltidium, and it is closely related to Acroihele. Yorkia differs from 

 Acroihele in its pronounced area, substance of shell, and place of origin of vascular trunks. 

 I do not know of any other genus of the Siphonotretidse with which it can be compared. 



The generic name is derived from York, Pennsylvania, near which the type species occurs. 



Yorkia ? miqueli n. sp. 



Plate LXXXII, figure 4. 



This species is represented by specimens of the ventral valve, that are compressed in 

 arenaceous shale, and distorted dorsal valves. The ventral valve is small, 2 to 4 mm. in 

 diameter; subcircular to transversely oval in outline, with the highest point at the apex, 



" Specimens from this locality are included in the collections of the United States National Museimi. 



