496 DR F. A. BATHER. 



convenient locality. But when the species are examined more closely we find, first, the 

 peculiar C. interruptus (§ 336), hitherto known only from somewhere in Baltic Russia ; 

 secondly, C. constrictus (§ 310), which seems not unlike the C. alter of Bohemia — a 

 species that is only a little older, but which also is more nearly allied to certain Baltic 

 species than to any from other parts of the world. 



§ 565. Entirely difi'erent are the relations of Pleurocystis (§§ 348, 390). Outside 

 Britain the genus is known only from the Ordovician of North America. The sandy 

 Ashgillian beds of Girvan, Wales, and Ireland furnish a group of species which have 

 attained a slightly more advanced stage of development. The correlation of these beds 

 with those of America is not easy. Though Pleurocystis and Cheirocrinus are 

 abundant in the Trenton Limestone, and though Mr W. K. Spencer tells me that he 

 recognises at least one Trenton form in the Asteroidea of the Starfish Bed, still there 

 can be no doubt as to the older age of the whole Trenton Group. Pleurocystis also 

 occurs in the Cincinnatian, which is generally taken to correspond with the Caradocian. 

 Dr R. S. Bassler, however, to whose discussion of the subject, based mainly on the 

 Bryozoa (1911), I am much indebted, places the upper part of the Lyckholm Limestone 

 in Richmondian time, which, with E. 0. Ulrich (1911), he refers to the Silurian. Prof 

 C. ScHUCHERT (1910) makes an independent period, the Cincinnatic, in which he includes 

 the Richmondian, but states (p. 532) that " on Anticosti . . . through 1134 feet of lime- 

 stones may be traced the gradual transition of the life of the highest Richmondian into that 

 of the earliest Siluric." It seems probable, then, that a large part of the Richmondian 

 corresponds to our Ashgillian. The correlation of isolated formations deposited during 

 a period of considerable shore-movement, such as characterised the close of the 

 Ordovician, must always be subject to uncertainty ; but, on the whole, we may conclude 

 that the earliest British Pleurocystis is a little younger than the youngest American 



§ 566. The Cystidea of the Starfish Bed tell us, then, that we have to account for 

 immigration from Bohemia on the one side and from Canada on the other, while we 

 must not exclude some connection with the north-eastern Baltic. Further, since the 

 species appear to have aff"ected a littoral, and probably even an arenaceous, habitat, the 

 connection must have been by way of a shore-line. This conclusion happily harmonises 

 .with the views held by those who have discussed these questions on a basis of wider 

 knowledge. " Conditions favoring littoral migration between England and the Gulf of 

 Saint Lawrence are indicated," says Ulrich (1911, p. 484), "rather commonly during 

 the Ordovician and Silurian. . . . The assumed land connection which made these 

 migrations possible served to separate the middle Atlantic fauna from the Arctic fauna 

 in the North Atlantic basin." In this connection it is interesting to learn that the 

 " Saint Clair limestone of northern Arkansas and eastern Oklahoma " — a pure crystalline 

 limestone of very early Silurian age — contains "a number of species that are closely 

 allied to Baltic, British, and Bohemian types" (Ulrich, 1911, p. 486). It is believed 

 that at the close of the Ordovician and in Silurian time, " Labrador, Greenland, and 

 Scandinavia were in a measure joined into one great land area, though perhaps with its 



