124 



PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



All the above 20 species are confined to the Llandeilo and to the 

 geographical areas assigned to them, and they are essentially cha- 

 racteristic ; the remaining 15 species pass to the Caradoc, and are 

 therefore of no value stratigraphically. To complete the analysis 

 of the distribution and value of the Crustacea I name the 15 species 

 that connect the Llandeilo and Bala formations : — 



Acidaspis Jamesii. 

 Homalonotus bisulcatus. 

 Illsenus Bowmanni. 

 Ampyx mammillatus. 



■ rostratus. 



nudus. 



Asaphus Powisii. 

 Calymene duplicata. 



Calymene Blunienbachii. 



brevicapitata. 



Trinucleus fimbriatus. 



concent ricus. 



Phacops apiculatus. 

 — — truneato-caudatus. 

 JEglina mirabilis. 



The singular Phyllopod Peltocaris aptychoides occurs both in Scot- 

 land and North Wales, and is not known out of the Llandeilo beds. 



Beaghtopoda. — This class is an important factor in the Llandeilo 

 rocks ; 10 genera and 34 species are known. Eive new genera appear, 

 Acrotreta, Crania, Rhynchonella, StropTiomena, and Leptcena ; the 

 first 4 with a single known species only ; but Leptcena has yielded 

 4 species, all of which occur in South Wales, and 2 of them in North 

 Wales. As we should anticipate, the species are most numerous in 

 South and North Wales. The former area gives us 26 species, the 

 latter 18 ; Scotland 9, and England 6. Although there are 20 species 

 in the Arenig group (and 34 in the Llandeilo) yet no species is common 

 to the two formations, no Arenig form passes up ; but of the 34 

 Llandeilo forms, 23 pass to the Caradoc. This is chiefly through 

 the genera Lingula, Orthis, and Leptcena — 4 out of 6 in the former 

 genus, 11 out of 13 in OrtJiis, and all the known 4 in Leptcena. 

 We have already seen that of the whole fauna of the Arenig (150 

 species) only 17 pass up into the Llandeilo ; whereas no less than 72 

 out of 175 Llandeilo species are found in the Caradoc, viz. Hydrozoa 

 16 species, Crustacea 17, Brachiopoda 23, Bivalvia 3, Gasteropoda 4, 

 Heteropoda 5, and Cephalopoda 2. The remaining forms are dis- 

 tributed through smaller classes. But for the distinctive character 

 of the Abereiddy-Bay and Caermarthenshire faunas, there is much 

 that is common between the Llandeilo and Caradoc. 



Lamellibeanchiata. — With the exception of the Irish species 

 Pleurorhynchus calcis, Baily, all the Bivalve Mollusca known are from 

 the South- Wales Llandeilos. We know of no Llandeilo Pelecypod 

 mollusk either in England or Scotland. Cardiola interrupta occurs 

 in both North and South Wales ; Modiolopsis expansa, M. inflata, 

 Ctenodonta varicosa, and Palcearca amygdalus only in the South- 

 Welsh beds. Three pass to the Caradoc. Pleurorhynchus and Car- 

 diola are new genera. 



Gasteeopoda. — South Wales has hitherto only given us 1 species, 

 Murchisonia simplex, M'Coy ; Ireland 2, Euomphalus parvus, Portl., 

 and Twsho tritorquatus, M'Coy ; the Shelve area and North Wales 

 Cyclonevia cr eon stria, M'Coy, and Euomphalus corndensis, Sow., the 



