152 PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Upper Llandovery and the distribution of its fauna, we are surprised 

 to find that, of the 261 species which occur in the formation, only 

 136 species really constitute the Upper-Llandovery fauna. The fact 

 that 104 of the species are Lower Llandovery goes very far to unite 

 the two so-called formations into one, or a Middle-Silurian group y 

 abolishing the terms Lower and Upper. Granting the high value of 

 the unconformity as of paramount stratigraphical importance, yet 

 the community of fossils and general facies of both horizons lead us 

 to regard the species as belonging to neither ; for certainly the Lower 

 Llandovery was an expiring close to the Caradoc and Bala, through 

 elevation of the Caradoc sea-bed, and the Upper through continuity 

 of certain species ; and the introduction of new forms commenced at 

 the base of the Upper Silurian of Sir R. Murchison and the Geological 

 Survey, from which, in the Wenlock rocks, were derived 58 genera 

 and 125 species ; for out of the 523 "Wenlock, 125 are Upper Llan- 

 dovery, 136 species only being the true Upper Llandovery fauna 

 proper. 



Plants. — One species only, a fucoid (Fucoides gracilis), is known 

 in this group of strata. This is probably only one of the many forms 

 of Annelide or molluscan tracks or burrows, which simulate the 

 habit of some of the marine algae. 



Pkotozoa. — ClatJirodictyon vesimlosum, Nich., and Cliona (Vioa) 

 gracilis constitute the only two Protozoa known. The former occurs 

 in the Llandovery rocks of Scotland ; the latter appears to be a 

 dichotomizing, burrowing sponge, found in the shelly structure 

 of a Pterincea (P. demissa) from the Upper Llandovery of the Mal- 

 vern Hills. This is probably the oldest known burroiving sponge : 

 its habit is quite that of the modern genus Cliona. 



The Welsh Llandovery rocks, although so carefully searched, have 

 yielded no traces of Spongidse. We should, however, quite expect 

 to find the group Silicipongia?, through the Hexactinellidaa, repre- 

 sented in these sediments or formations accumulated in moderately 

 deep water, and associated with an extensive Ccelenterate and 

 Brachiopodal fauna, such as we have in the Upper Llandovery. The 

 Analytical Table (p. 159) shows that only in two of the ten 

 (Worcestershire and Scotland) are the Protozoa known. 



Hydkozoa. — The genera Monograptus, Cyrtograptus, Diplograptus, 

 and JRetiolites alone seem to represent the Rhabdophora in the Upper 

 Llandovery rocks ; but so many species appear to be common to the 

 Lower and Upper Llandovery, that it is with difficulty I am enabled 

 to realize the distinctness of the two faunas. The Irish, Scotch 

 (Girvan, Gala, and Mulloch beds), and the Coniston flags and mud- 

 stones have so many species in common with the Lower Llandovery 

 that only through intimate and practioal acquaintance with the two 

 groups of species can they be separated. In the Yalentian or 

 Llandovery-Tarannon series of Lapworth, including the Birkhill 

 series and its several zones, so many species of Monograptus, Diplo- 

 graptus, and Cyrtograptus appear to be common, that it is no easy 

 task to determine numerically or statigraphically, especially when 

 geographical distribution is also taken into consideration, their true 



