BRACHIOPODA. 421 
Dinorthis.] 
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 O. 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 O. 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 O. pectinella, but differing in the 
far greater number of striz. At that time no form was known to connect the sub- 
genera Dinorthis and Plwsiomys, respectively typified by O. pectinella and O. subquad- 
rata. In the Lower Silurian of Minnesota, beginning with the lowest fossiliferous 
member of the Trenton limestone, the first species is O. 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 
strie, while in O. pectinella there are strong plications. From 0. pectinella we pass 
to the variety sweeney?, 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 strie prevail. Upon reaching the strata containing 
Clitambonites diversa Shaler, provisionally known to the survey as Galena shales, the 
numerously striated form, here described as O. meedsi, is the only one found. Ascend- 
ing into the Galena formation for thirty or forty feet more we find O. meedsi still 
exhibiting a tendency to increase the number of its striz, and finally assuming char- 
acters (variety germana) which attain their greatest development in O. subquadrata 
of the Hudson River formation. The change from O. pectinella to O. subquadrata is 
thus completed. 
The cardinal process is linear and not much elevated in O. deflecta, and attains 
its greatest development in O. subquadrata and O. proavita of the Hudson River 
formation. The adjustor scars are conspicuous in O. 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) subequata has sometimes a short deltidium, the 
