ii4 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



genera. Eleven new genera of Crustacea, 10 of which are Trilohita, 

 appeared in the Arenig sea: they are yEglina, Angelina, Trinu- 

 cleus, Barrandia, Galymene, Phacops, Placoparia, Lllcenus, Illce- 

 nopsis, Homalonotus, and the Phyllopod Caryocaris, which, with 

 Placoparia, never passed to higher rocks. 



Brachiopoda. — 7 genera, Lingulella, Lingula, Orthis, Obolella, Pis- 

 cina, Siphonotreta, and Pinobolus, represented by only 18 species, 

 are all that can be recorded of this class through the Arenig rocks. 

 All the genera and 12 of the 18 species occur in the St.-David's area. 

 Only 2 species are known in the Skiddaw rocks, Lingula brevis and 

 a Piscina ; and 5 in the beds west of the Stiperstones ; they are 

 Lingula attenuata, Orthis alata, 0. calligramma, 0. striatula, and 

 Obolella plumbea. The 5 North-Wales forms are Lingulella Pavisii, 

 L. lepis, Orthis lenticularis, 0. calligramma, and Obolella plicata, 

 Tai-hirion yielding 4 of the 5. Lingula petalon, L. brevis, and 

 Orthis calligramma are all that pass to the Llandeilo rocks ; but 9 

 come from the Tremadoc ; therefore the special forms are only 6 — 

 Siphonotreta micula, Pinobolus Hicksii, Dav., Piscina sp., Orthis 

 striatula, 0. remota, and 0. alata. The new genera are Pinobolus 

 and Siphonotreta. 



Lamellibranchiata. — Ribeiria complanata, Modiolopsis trapezi- 

 formis, Palmarca amygdalus, and Redonia anglica occur in the 

 Shelve Arenigs. Paloearca socialis and a species of Ctenodonta occur 

 only at Ty-obry in North Wales. None are known either in the 

 Skiddaw rocks or in South Wales, 6 species being the entire bivalve 

 fauna. Pibeiria and Redonia first appear in the Arenig rocks. 



Gasteropoda. — Only 4 genera and 4 species are at present known. 

 Pleurotomaria llanvirnensis, Hicks, and Ophileta sp., occur at 

 Llanvirn, St. David's ; Rhaphistoma, Skiddaw only ; and the Stiper- 

 stone form is Euomphalus corndensis, Sow. No species known in 

 North Wales. 



Pteropoda. — 4 species of Conidaria and 4 of Theca constitute 

 the Arenig Pteropod fauna. The Shelve species are Theca simplex, 

 Salt., and T. vaginula, Salt. Those occurring in North Wales are 

 Theca vaginula, Salt., Conidaria corium, Salt., and C. margariti- 

 fera, Salt. Theca Harhnessii, Hicks, T. caereesiensis, Hicks, Conularia 

 llanvirnensis, Hicks, and C. Homfrayi, Salt., range through the St.- 

 David's Arenigs. 



Heteropoda. — 4 species of Bellerophon in the Stiperstone beds 

 and 2 in the St.-David's are all that are known. The Stiperstone 

 species are B. hippopus, Salt., B. bilobatus, Sow., B. carinarioides, 

 and B. perturb atus, Sow. The South-Wales species are B. multi- 

 striatus, Salt., and B. llanvirnensis, Hicks. No species yet known in 

 North Wales. 



Cephalopoda. — Orthoceras Avelinii and 0. encrinale occur in the 

 Corn don area (Shropshire). 0. sericeum and caereesiense are only 

 known at St. David's, South Wales ; an undeterminable form occurs 

 at Ty-obry (Tremadoc, North Wales). None in the Skiddaw beds. 



The distinctive value of the Arenig fauna is shown in the fact 

 that only 8 genera and 9 species pass to the Llandeilo ; and 4 



