Trinucleus ornatus, Burm.
Cystideæ.—Among the additions which recent research has made to the paleontology of the oldest Silurian rocks, none are more remarkable than the radiated animals called Cystideæ. Their structure and relations were first elucidated in an essay published by Von Buch at Berlin in 1845. They are usually met with as spheroidal bodies covered with polygonal plates, with a mouth on the upper side, and a point of attachment for a stem b (which is almost always broken off) on the lower. (See [fig. 433.]) They are considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Sphæronites here represented ([fig. 433.]) occurs in the Llandeilo beds in Wales.[358-A]
Fig. 433.
Sphæronites balticus, Eichwald. (Of the family Cystideæ.)
- a. mouth.
- b. point of attachment of stem.
Lower Silurian, Shole's Hook and Bala.
Thickness and unconformability of Silurian strata.—According to the observation of our government surveyors in North Wales, the Lower Silurian strata of that region attain, in conjunction with the contemporaneous volcanic rocks, the extraordinary thickness of 27,000 feet. One of the groups, called the trappean, consisting of slates and associated volcanic ash and greenstone, is 15,000 feet thick. Another series, called the Bala group, composed of slates and grits with an impure limestone rich in organic remains, is 9,000 feet thick.[359-A]