614 CAMBKIAN BRACHIOPODA. 



Genus DEARBORNIA Walcott. 



Dearbornia Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 78. (Mentioned as below aa a new genus.) 

 Dearbornia Walcott, 1908, idem, vol. 53, No. 4, PL XI, and pp. 142 and 146. (Classification of genus.) 



This genus is based on one species, which is well represented by fourteen specimens. The 

 generic description is incorporated with the description of the type species. 



Type. — Dearhornia clarki Walcott. 



The generic name is taken from Mount Dearborn, named after Gen. Henry Dearborn, 

 near which the type specimens were collected. 



, Dearbornia clarki Walcott. 



Plate LXXXII, figures 7, 7a-d. 



Dearbornia clarki Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 78-80, PI. VIII, fig. 7. (Described and 

 discussed as below as a new species. Fig. 7 is copied in this monograph, PI. LXXXII, fig. 7.) 



Shell subequivalye, subcircular in outline, slightly convex. Ventral valve most elevated 

 at the pedicle aperture, which is circular, rather large, and situated from one-fifth to one-sixth 

 the length of the valve from the posterior margin; the slope back of the foramen is gently 

 rounded and without a trace of false area or pedicle groove; the position of the beak is not clearly 

 defined, as the margin is rounded and the uniform slope of the outer surface is unbroken. Dorsal 

 valve uniformly and slightly convex; the position of the beak is indicated by a shght projection 

 of the outline of the valve. 



Surface marked by fine concentric lines. The substance of the shell is calcareous in an 

 oolitic limestone in which semiphosphatic shells of Oholus are preserved. The shell is thick and 

 apparently formed of one layer, but this is probably, as in the case of the shells of Oholella 

 crassa (Hall), a condition of preservation, the original layers or lamella? having been replaced or 

 else cemented together. The average size of the valves is from 3 to 5 mm. 



The interior of the ventral valve does not show a true area; there is a space between the 

 margin and the end of the median furrow, into which the foramen opens. The median furrow 

 is rather broad and deepest at the foramen; it extends forward beyond the center of the valve; 

 the furrow into which the foramen opens is broadest at the posterior end, and runs out to a 

 point a little in advance of the opening; from each side of the furrow and opposite the opening a 

 furrow extends obhquely outward and then forward subparallel to the median furrow. Two 

 large, oval muscle scars occur in the space between the outer fiuTOw and the posterolateral 

 margin of the shell; these scars correspond in position to the transmedian and anterior lateral 

 muscle scars of Oholus and Trematoholus. Nothing is clearly shown of the position of the main 

 vascular canals unless the grooves outside of the median depression indicate their position, or 

 it may be that they were on the narrow ridges outside of the side furrows and inside of the 

 lateral muscle scars. 



The interior of the dorsal valve shows a rudimentary area much like that of Rustella edsoni 

 Walcott (PI. I, fig. Ic) ; the area is a smooth space, with a slightly defined central depression 

 from which a narrow, low median septum extends forward to about the center of the valve; a 

 narrow ridge extends forward from the posterior central depression on each side at about the 

 inner third of the distance between the median septum and the outer margin; these ridges 

 probably marked the position of the main vascular sinuses. The central muscle scars (h) 

 occur in the shallow depression on each side of the median septum, a little back of the trans- 

 verse center of the valve, and the transmedian scars and outside laterals are just outside of 

 the narrow ridges on each side of the valve; these scars, like those in the ventral valve, are 

 large for so small a shell. 



Observations. — Dearbornia clarTci is one of the simple or rudimentary forms of Siphono- 

 tretidae. It differs from Siphonotreta in the absence of an area and a siphonal or pedicle tube, 

 in having the pedicle opening on the umbo in advance of the beak, and in its calcareous shell. 

 The circular pedicle aperture ^¥ithout an exterior furrow, the absence of a well-defined area on 

 the ventral valve, and its calcareous shell distinguish it from Trematobolus and ScJiizambon. 

 The form and position of the pedicle opening suggest Discinopsis, but the interiors of the valves 



