I48 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Crustacea. — The Crustacea of the Lower Llandovery reveal to us 

 numerically only 2 species in each genus. We have much yet to 

 learn of the history and zoological relations of the Trilobita of the 

 Llandovery rocks. The 13 genera and 25 species of this order 

 mostly characterize the rocks of South Wales and Scotland ; 2 genera 

 and 4 species only are known in North Wales, Calymene (3) and 

 Trinucleus (1) ; and 7 genera and 8 species in South Wales ; Scotland 

 has 9 genera and 16 species. Eighteen of the Trilobita are also Cara- 

 doc ; and 8 genera and 10 species pass to the Upper Llandovery ; so 

 that in the Lower Llandovery one of its largest groups has little 

 or no stratigraphical value in classification. 



Brachtopoda. — This class, like the Crustacea, has little or no classifi- 

 catory value ; for of the known 10 genera and 59 species in the Lower 

 Llandovery, 8 genera and 35 species are also Caradoc and Bala, and 

 10 genera and 38 species pass to the Upper Llandovery. The charac- 

 teristic species are Meristella suhundata, Orihis Bouchardi, Rhyncho- 

 nella tripartita, Spirifera exporrecta, StropJiomena arenacea, StricJc- 

 landinia lens, Pentamerus oblongus, and P. undatus. The first ap- 

 pearance of Pentamerus and StricMandinia is a marked and im- 

 portant feature in the Lower Llandovery beds. Three species of 

 Pentamerus (P. oblongus, P. globosus, and P. undatus) are Lower and 

 Upper Llandovery forms ; the 2 species of StricJclandinia, S. lens 

 and S. lirata, are common to both horizons. In North Wales, 8 

 genera and 22 species occur ; and in South Wales 10 genera and 48 

 species ; Scotland has yielded 8 genera and 29 species, and Ireland 

 9 genera and 27 species. The South-Wales beds are enriched 

 chiefly through the genus Orthis with 20 species, Strophomena with 

 11, and Bhynchonella with 7. 



Lamellibranchiata. — No species of this extensive group of Mol- 

 lusca occurs either in North Wales or in Scotland ; and we are only 

 acquainted with 3 genera and 3 species in the Lower Llandovery, 

 viz. Pterinosa retroflexa, Mytilus mytilimeris, and Ortlionota sulcata-, 

 these 3 are South-Wales forms, and the Pterincea occurs in the Irish 

 Llandovery. This meagre representation of so large a class in these 

 beds, as compared with the Caradoc with 16 genera and 76 species, 

 clearly shows that at the close of the Caradoc and Bala period great 

 changes took place with regard to the relations of land and sea. The 

 Caradoc sea evidently shallowed and reduced its area, and this was 

 accompanied by corresponding sedimentary changes which governed 

 the life, habits, and distribution of the succeeding Llandovery fauna. 

 This is borne out by the Brachiopoda and other groups which 

 reached such perfection in the Caradoc seas (109 species), Crustacea 

 (146 species), the Lamellibranchs (76 species), Gasteropoda (53 

 species), and Cephalopoda (47 species) ; whereas in the Lower Llan- 

 dovery many of these groups are scarcely represented ; for example, 

 the group we are now treating of has only 3 species, the Echino- 

 dermata 2, theGasteropoda 1 6, and the Cephalopoda only 3, with 

 only 1 species of Pteropod and 1 Heteropod. 



Doubtless the strictly arenaceous nature of the deposits and the 

 somewhat shallow sea constituted the main cause of this extremely 



