582 PERIOD OF INVERTEBRATE ANIMALS. [Ch. XXVII. 



lated bands of magnesian limestone, and in their lower some argilla- 

 ceous beds. Among the shells of these strata are species of Lingula 

 and Orthis, and several trilobites of the new genus Dikelocephalus 

 (fig. 671). These rocks, occurring in Iowa, 

 Fig. 671. Wisconsin, and Minnesota, seem destined 



hereafter to throw great light on the state 

 of organic life in the Cambrian period. 

 Six beds containing trilobites, separated by- 

 strata from 10 to 150 feet thick, are already 

 enumerated. 



I have seen the Potsdam sandstone on 



the banks of the St. Lawrence in Canada, 



and on the borders of Lake Champlain, 



where, as at Keesville, it is a white quartz- 



ose fine-grained grit, almost passing into 



quartzite. It is divided into horizontal 



m7 DSe P Owen J ^mTT iS ' ri PP le " mar ked beds, very like those of the 



a large crustoTean'of tiToienoid Lingula flags of Britain, and replete with a 



group. Potsdam Sandstone, small round-shaped Lingula (Obolella of 



Falls of St. Croix, on the upper t-,.,,. \ . -. -. . ,. . ^ .-, 



'! Mississippi. -Billings), m such numbers as to divide the 



rock into parallel planes, in the same man- 

 ner as do the scales of mica in some micaceous sandstones. This 

 formation, as we learn from Sir W. Logan, is 700 feet thick in Cana- 

 da ; the lower portion consisting of a conglomerate with quartz peb- 

 bles ; the upper part of sandstone containing fucoids, and perforated 

 by small vertical holes, which are very characteristic of the rock, and 

 appear to have been made by annelids [Scolithus linearis). 



On the banks of the St. Lawrence, near Beauharnois and elsewhere, 

 many fossil footprints have been observed on the surface of its rippled 

 layers. These impressions were first noticed by Mr. Abraham, of 

 Montreal, in 1847, and were supposed to be tracks of a tortoise; but 

 Sir W. Logan brought in 1851 some of the slabs to London, together 

 with numerous casts of other slabs, enabling Professor Owen to cor- 

 rect the idea first entertained, and to decide that they were not due 

 to a chelonian, nor, as he imagines, to any vertebrate creature. The 

 Professor inclines to the belief that they are the trails of more than 

 one species of articulate animal, probably allied to the King Crab, or 

 Limulus. Between the two rows of foot-tracks runs an impressed 

 median line or channel, supposed by the Professor to have been made 

 by a caudal appendage rather than by a prominent part of the trunk. 

 Some individuals appear to have had three, and others five pairs, of 

 limbs used for locomotion. The width of the tracks between the 

 outermost impressions varies from 3-J- to 5|- inches, which would 

 imply a creature of much larger dimensions than any organic body 

 yet obtained from strata of such antiquity. In this respect they agree 

 with the gigantic Eurypteridce, detected in the Lowest Devonian 

 and uppermost Silurian rocks. Their size alone is important, as 



