Ch. XXVII.] PERIOD OF INVERTEBRATE A:N"IMALS. 



453 



Bikelocephalus Minnesotenis^ 



Dale Owen. 5 diameter. 

 A large crustacean of the Glenoid 



group. Potsdam Sandstone. 



Falls of St. Croix, on the upper 



Mississippi. 



sedimentary rocks near the head-waters of the Fig. eia 



Mississippi, lying at the base of the whole 

 Silurian series. They are many hundred feet 

 thick, and for the most part similar in char- 

 acter to the Potsdam Sandstone above de- 

 scribed, but including in their upper portions 

 intercalated bands of magnesian limestone, and 

 in their lower some argillaceous beds. Among 

 the shells of these strata are species of Lingula 

 md Orthis, and several trilobites of the new 

 genus Dikelocephalus (fig. 618). These rocks, 

 occurring in Iowa, "Wisconsin, and Minnesota, 

 seem destined hereafter to throw great light 

 on the state of oi'ganic hfe in the Cambrian 

 period. Six beds containing trilobites, sepa- 

 rated by strata from 10 to 150 feet thick, are already enumerated. 



Relation of Siluriari and Cambrian Faunas. — That there is a con- 

 siderable connection between the Cambrian and lower Silurian faunas, 

 notwithstanding that nearly every species may be distinct, seems evident ; 

 but it may not be a closer one than that existing between the Upper 

 Silurian and Devonian. This I infer from the following facts, — that in 

 Bohemia, where the Cambrian or primordial fauna of Barrande is best 

 developed, it consists mainly of Trilobites ; and of this order more than 

 two-thirds of the genera and all the species, more than twenty in number, 

 are, with one exception (Agnostus pisiformis), distinct from the Silurian. 

 But M. Barrande observes that out of thirty-nine Silurian genera of 

 Trilobites, no less than eleven pass upwards into the Devonian. If, there- 

 fore, we had only trilobites in the latter, its generic relationship to the 

 Silurian fauna would appear greater than that of the Silurian to the Cam- 

 brian. And, though the details of the English rocks of this age are not 

 yet fully known, the species at least appear all to be distinct. The same 

 holds good with regard to the fossils of the Swedish strata, and, as we 

 have seen, to those of America. 



A distinctive character, therefore, is given to the fauna of this period, 

 by which we seem to be carried one step farther back into the history of 

 organic life. 



Supposed Period of Invertebrate Animals. 

 "We have seen that in the upper part of the Silurian system a bone-bed 

 occurs near Ludlow, in which the remains of fish are abundant, and 

 amongst them some of a highly organized structui-e, referred to the genus 

 Onchus. We are indebted to Sir R. Murchison for having first an- 

 nounced, in 1840, the discovery of these ichthyolites, and he then spoke 

 of them as " the most ancient beings of their class." In his new and 

 excellent work, entitled " Siluria" (p. 239), he reverts to the opinion 

 formerly expressed by him, and observes that the active researches of the 

 last fourteen years in Europe and America " have failed to modify that 



