164 AMERICAN JOURNAL 



gined and somewhat produced ; channelled by a narrow trian- 

 gular groove, the anterior end of which is flanked within by two 

 very prominent, thick, conical, shelly bosses ; representing hinge 

 teeth. Substance of the valve thick, testaceous, not glossy ; 

 minutely fibrous but not distinctly punctured under a lens of 

 moderate power except by the end of the fibres (McCoy). 



Type. jSjJondi/lobus craniolaris, McCoy, An. Nat. Hist. 1851, 

 p. 408. 



Hab. Cambro-Silurian, McCoy. 



Note. In the 16th Regents Report (1863), Prof. Hall figures, 

 under the name of Oholella ? polita, a shell which he described as 

 Lingulaf polita, in the Ann. Rep. Geol. Wisconsin, p. 24, 1850. 

 He compares it with Spondylobiis and Obolella, and doubtfully 

 refers it to the last named genus. Owen gives an unrecogniz- 

 able figure of the same shell under the name of Obolus (Appo- 

 linus ?) in his Geol. Rep. of Wiscon., Iowa and Minn., pi. i, B, 

 fig. 9, 11, 15, 20. Prof. Hall describes the muscular impressions 

 as follows : " The interior of the ventral ? (neural) valve has a 

 broad, somewhat cordiform, subcentral duplicate muscular im- 

 pression, with a raised margin. From the centre beneath the 

 beak extends a low flatly rounded ridge, which reaches into and 

 partially divides the muscular impression. On each side of this 

 low ridge is a nearly flat, slightly elevated plate or thickening 

 of the shell, which, extending forward, is continued in its outer 

 limbs in a raised border nearly around the muscular impression ; 

 giving to the whole a broad spatulate form, leaving a space in 

 front where the shell is marked by a few radiating or vascular 

 impressions. On each side of the central elevated plate and at 

 the anterior extremities of the cardinal line, are two smaller 

 muscular impressions. There is a nazTOw groove beneath the 

 apex of the valve and on each side an elongate thickened boss 

 or pad." (That is to say the hinge line is broad, arched and 

 thickened, with a mesial groove for the peduncle). " The dor- 

 sal ? (haemal) valve has a narrow central muscular impression, 

 the limits of which extend in an acute point below the middle of 

 the shell ; on each side is a broader limited area, reaching two- 

 thirds the length of the shell, and extending into 'the cavity 

 beneath the beak. The whole presents an ovate, somewhat 

 cordiform area. No marginal muscular impressions have been 

 observed in this valve. Apex emarginate, with a prominent pad 

 (i, e. the thickened hinge line) or tooth-like process on either 

 side." Hab. Lowest fossiliferous beds of the Potzdam sand- 

 stone (Hall). 



The above description is not as clear as might be desired and 



