HABITAT. 159 



Detailed stratigrapMc distribution of Cambrian and Ordovician Brachiopoda — Continued. 



Vermont. 



The localities in Vermont are somewhat scattering, but most of them are confined to a fairly well-marked horizon 

 in the Lower Cambrian. Two (319f and 227) occur in the Ordovician, tour (28, 16n, 87', and 319c() in the Upper Cam- 

 brian, and two (28a and 319s) in the Middle Cambrian. The interrelations o£ these localities (see index for num- 

 bers) are obscure and" stratigraphic comparisons would have little value. 



Virginia and Maryland. 

 All but two of the localities in Maryland and Virginia (see index for numbers) occur in the same Lower Cambrian 

 sandstone horizon. One Upper Cambrian locality (92b) and one Middle (47h) have been identified. 



Wales. 



See England, Scotland, and Wales, where the different horizons are more or less closely differentiated and corre- 

 lated, with complete lists of localities and included species. 



Wisconsin, Michigan, Minnesota, and Iowa. 



Two localities (339 and 364a) from the Lower Ordovician Oneota dolomite and three (328b, 339a, and 364) from the 

 Upper Cambrian St. Lawrence formation have been identified by Sardeson [1896, pp. 95 and 96]. The remainder are 

 from the "St. Croix sandstone," and have all been referred to the Upper Cambrian, with the following exceptions: 

 79x, 83^ 84, 84f, 84s, 328e, 328g, 339e, 339i, 339j, and 339k. These eleven localities occur in the lower portion of the 

 "St. Croix sandstone" and while they may belong in the Upper Cambrian, they have been placed in the Middle 

 Cambrian because of the entire absence in the collections at our disposal of characteristic Upper Cambrian forms. 



Wyoming. 



See both South Dakota and Montana for general discussion of stratigraphy. 



HABITAT. 



The conditions in which the Cambrian brachiopods are found indicate that some of them 

 were gregarious in habit, and that many persisted through marked changes of environment and 

 sedimentation. Micromitra (IpJiidella) pannula, for instance, is found in sandstone, siUceous 

 and argillaceous shale, and limestone. It has a wide distribution in the.Cordilleran province of 

 western North America, and has a vertical range of 2,000 feet or more. Other forms, such as 

 Micromitra Tiaydeni, are known only from one locality and one layer of rock. A large 

 number of species occur in sandstone and shales that are evidently of shallow- water origin; 

 others occur in limestones that were probably deposited in relatively deep water. The evidence 

 indicates that their habitat largely ranged from between tides to a depth of 1,000 to 2,000 feet. 

 Some forms may have had a greater bathymetric range, but the evidence in favor of such a 

 conclusion is not known to me. 



In the following list there has been brought together a summary statement of the character 

 of the sediment in which each of the genera taken up in this monograph occurs. Gradations 

 froin one type of sediment to another are so frequent and close that some of the separations may 

 be considered arbitrary, though most of the determinations have been made by an examination 

 of the hand specimen upon which the species is preserved. The numbers in the columns give 

 the number of species occurring in the different classes of sediment and the number which have 

 been identified from more than one class. The totals at the end of the table give the number of 

 species and the number of genera, respectively, that have been identified from the different 

 sediments. 



It is probable that a reexamination of some of the specimens would justify their transfer to 

 another column; for instance, the one sandstone representative of the genus Huenella might be 

 found to occur in a calcareous sandstone or arenaceous limestone, which might equally as well 

 be placed in the limestone column; but the table as a whole and the figures for each genus seem 

 to indicate that the various genera are by no means confined to nor even characteristic of a given 

 type of sediment. On the other hand, a similarly prepared table of the species in the mono- 

 graph showed that with few exceptions each of the species is confined to one type of sediment. 

 Disregarding those species (about 200) which have been found at but one locality, the table 

 showed that out of over 500 specific occurrences only 150 had been identified from more than 

 one type of sediment. Errors in the identification of sediment would be of more importance in 

 the latter case than in that of the genera, but none of the specimens were reexamined and in 

 none of the tables has there been any rearrangement of the figures. 



