444 THE PALEONTOLOGY OF MINNNSOTA. 



[Orthis (Dalmanella) testudlnaria 



collectors have almost lost sight of the original species. Some of these varieties 

 seem to mark given horizons over limited areas, and others apparently characterize 

 special localities; still, I do not believe they are sufficiently distinct or persistent 

 enough to rank as species or to be worthy a varietal name beyond the purpose of 

 the locality where found, or for local preservation." (Whitfield, loc. cit.) Specimens 

 of 0. testudinaria from the Trenton shales and from the lower portion of the Galena 

 formation of Minnesota are slightly smaller and narrower than those from the 

 Trenton of New York, while in central Kentucky they are generally twice the size 

 of the eastern examples. Other specimens from the latter region are very thin-shelled 

 and are referred by local collectors to 0. emacerata Hall. In the Cincinnati group of 

 the Ohio valley, 0. testudinaria is abundant in certain horizons. It is, moreover, 

 nearly always present in one form or another throughout the formation and is more 

 or less variable, although constant in its characters in certain beds. Some varieties 

 are known as 0. cyclus James, 0. multiseda (James) Meek, 0. emacerata Hall, 0. jugosa 

 James and 0. meeki Miller. The first two are synonymous and cannot be regarded as 

 of greater significance than a local variation; the same is also true of the last two. 

 0. emacerata can be distinguished from 0. testudinaria by its tb.in, compressed valves 

 and finer and more numerous striae. If, however, a close examination is made 

 between specimens of 0. testudinaria from various localities it will be apparent to 

 the observer that individuals from one region do not exactly agree with those from 

 another. As long as one restricts himself in his studies of this species to a single 

 horizon of one locality all goes well with the selected varieties, but as soon as the 

 trial is made to apply them to specimens from other regions the chosen varieties 

 drop out for want of constancy of characters. 



In some Minnesota specimens there is a tendency to greater convexity of the 

 dorsal valve. The mesial sinus may be obsolete or, as seen in one specimen, 

 narrow and deep, sharply indenting the anterior margin. Probably if the develop- 

 ment of the sinus were to become deeper and deeper in successive generations, it 

 would eventually result in a species related to Bilobites. The general expression of 

 0. testudinaria and species of Bilobites, excluding the strongly lobate condition of the 

 latter genus, is essentially the same. While this may be the line of development, 

 still Bilobites may have originated from an entirely different stock. Dr. Beecher, in 

 his " Development of Bilobites," has suggested its relations to the group represented 

 by Platystrophia biforata Schlotheim.* 



Formation and locality. Not common in the Trenton shales at Minneapolis, St. Paul, Cannon 

 Falls, Chatflcld, Preston and elsewhere in Minnesota; Decorah and McGregor, Iowa. Very common in 

 the Galena shales at various localities in Goodhue and Olmsted counties, Minnesota. Also from the base 

 of the " Upper Ruff beds" of the Trenton at Mineral Point, Wisconsin, and Rockton, Illinois. From the 

 Galena at Decorah and Dubuque, Iowa; Neenah and Oshkosh, Wisconsin. In the Trenton limestone of 



American Jour. Sci., vol. xlii, p. 54. 1891. 



