BRACHIOPODA. 421 



Dtnortbis.] 



apical callosity, but it is never carried as far as in the forms mentioned in the group 

 of Orthis callactis, and, so far as observed, its existence is confined to the species 

 cited. In the brachial or more convex valve the area is narrower, the Crural plates 

 stronger than in the preceding groups [Orthis callactis and 0. plicatella], and the 

 cardinal process, instead of being a simple linear ridge lying in the bottom of the 

 deltidial cavity, is an erect apophysis, broadened and frequently bilobed on its sum- 

 mit and posterior face. 



"The shell structure, like that of Orthis callactis and 0. plicatella, is compactly 

 fibrous and, in all the species examined, impunctate. No evidence of tubulose plica- 

 tions has been seen." 



Type: Orthis pectinella Emmons. 



In establishing the subgenus Dinorthis Prof. Hall was justified in separating the 

 species having a similar contour of the valves, as in 0. pectinella, but differing in the 

 far greater number of striae. At that time no form was known to connect the sub- 

 genera Dinorthis and Plcesiomys, respectively typified by 0. pectinella and 0. subquad- 

 rata. In the Lower Silurian of Minnesota, beginning with the lowest fossiliferous 

 member of the Trenton limestone, the first species is 0. deflecta. This has characters 

 common to the strophomenoids, but is clearly referable to Dinorthis. From this 

 species to 0. (D.) pectinella, the one occurring next higher in the series, in the Trenton 

 shales, there is a wide departure in the surface striation. 0. deflecta has very fine 

 striae, while in 0. pectinella there are strong plications. From 0. pectinella we pass 

 to the variety sweeneyi, which is a local variation of it. Associated with the latter are 

 specimens in which the strong plications begin to divide near the anterior margin. 

 At the base of the Galena shales the strong, simple, plicated forms become rarer, 

 while those with more numerous striae prevail. Upon reaching the strata containing 

 Clitambonites diversa Shaler, provisionally known to the survey as Galena shales, the 

 numerously striated form, here described as 0. meedsi, is the only one found. Ascend- 

 ing into the Galena formation for thirty or forty feet more we find 0. meedsi still 

 exhibiting a tendency to increase the number of its striae, and finally assuming char- 

 acters (variety germana) which attain their greatest development in 0. subquadrata 

 of the Hudson River formation. The change from 0. pectinella to 0. subquadrata is 

 thus completed. 



The cardinal process is linear and not much elevated in 0. deflecta, and attains 

 its greatest development in 0. subquadrata and 0. proavita of the Hudson River 

 formation. The adjuster scars are conspicuous in 0. deflecta, while in the other 

 species here referred to Dinorthis they are much reduced in size. 



In Orthis (Dinorthis) deflecta no pedicle muscle exists, but always a more or less 

 large deltidium. Orthis (Dalmanella) subcequata has sometimes a short deltidium, the 



