BRACHIOPODA. 417 



Ortbldee.] 



margin, where they become obsolete. The depression between this ridge and the 

 outer elevated margin of the muscular area is granulose, while the slope on the 

 outer side of the former is quite abrupt. 



This small species of Plectambonites is quite distinct from all other American 

 forms. The convexity of the shell, surface ornametation and the interior char- 

 acters of the dorsal valve will distinguish it at once from P. sericea Sowerby and 

 P. decipiens Billings. Its relationship is rather with the latter species, on account 

 of the sharp elevation just inside the margin of the dorsal valve. P. gibbosa seems 

 to be closely allied to P. quinquecoslata McCoy,* but until more is known of the 

 interior characters of the latter further comparisons a*re impossible. That species 

 is found in the Caradoc and Llandovery of England; also in Scotland, Ireland and 

 Russia. 



Formation and locality. Not uncommon in the Galena at several localities in Goodhue county; also 

 at Mantorville and Old Concord, Minnesota. 



Collectors. M. W. Harrington, A. D. Meeds, W. H. Scofleld, E. O. Ulrich and the writers. 

 Mus. Reg. Nos. H7, 423, 8165, 8166. 



Family ORTHID^}, Woodward. 

 Genus ORTHIS, Dalman, emend Hall. 



1828. Orthit, DALMAN. Kongl. Vet. Acad.-Hand., pp. 93, 96. 



1892. Orthis, HALL. Palaeontology of New York, vol. viii, pt. i, p. 192. 



Description: "The distinguishing features of these shells are the plano-convex 

 contour; the strong, sharp and comparatively few costse, rarely, if ever, bifurcating; 

 the elevated and somewhat incurved cardinal area on the pedicle valve; the relatively 

 slight development of the dental lamellae, which do not extend the entire length of 

 the umbonal cavity. The cardinal process on the brachial valve is an elongate, 

 vertical plate, extending from the apex the whole length of the delthyrium, thus 

 longitudinally dividing the deep deltidial cavity. It is usually simple, both on the 

 outer edge and at its distal extremity. 



"In this group of orthids [Orthis restricted], more frequently than elsewhere, we 

 find a character rarely developed in any stage of growth, viz: the existence of a 

 transverse apical plate in the delthyrium of the pedicle valve [the rudiments of a 

 deltidium]. The greatest development attained by this feature, in 



any of the numerous species of Orthis studied, is to be found in 0. tricenaria of the 

 Trenton and Hudson River faunas; it has also been observed in 0. calligramma, var. 

 davidsoni, although it does not appear in any of the figures of this species and its 

 varieties given by Mr. Davidson, nor is any mention made of it in his descriptions. Its 



*Sil. Foss, Ireland, p. 33. pi. m. fl. 8, 1846; and Davidson's Mono. British 811. Brach., p. 822, pi. xi.vm, figs. 23-27. 

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