66 



PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



C. solvensis), 3 species of Leperdiiia (L. ferruginea, L. cambrensis, 

 and L. primcevci), Microdisczis sculptus, and Plutonia Sedgwiehi 

 are all confined to the Harlech and Llanberris group of St. 

 David's. They constitute the Crustacean fauna of that horizon, 

 out of 103 species known in the six divisions of the Lower Cam- 

 brian rocks, and up to the close of the Upper Tremadoc. The Me- 

 nevian group possesses the largest Crustacean fauna in the eleven 

 classes, and, except the Brachiopoda, the largest number indi- 

 vidually. 



Hydrozoa. — No form known below the Upper Lingula-flags or 

 the Lower Tremadoc, from which Dr. Callaway has obtained the 

 earliest known species, Bryograptus Callavei, Lapw. Hr. Lap- 

 worth's genus occurs in the Shineton shales of Shropshire. The 

 same genus occurs in the OZemts-beds of "Westrogothia, in Sweden, 

 Linnarsson having detected these oldest Khabdophora in that area. 



Brachiopoda. — Only 6 species are essentially Lower-Cambrian 

 or Harlech forms, Discina pileolus, D. caerfaiensis, Lingulella ferru- 

 ginea and var. ovalis, L. primoeva^ and Orthis saglttalis ; this last 

 species is doubtful. Five genera and 20 species range through the 

 six horizons, or from the Longmynd group to the Upper Tremadoc ; 

 but, as we have seen, only the 6 just named occur in the lowest 

 horizon ; 4 of these belong to the Tretenterata (non-articulate group). 

 It is doubtful if the others occur in the Longmynd group. 



Lamellibranchiata. — No bivalve mollusk is known to occur 

 below the Lower Tremadoc. In that formation for the first time 

 in Britain we recognize 5 genera with 12 species. Davidia and 

 Glyptarca are new ; the remaining 3 are the well-known genera 

 Modiolopsis, Palcearca, and Ctenodonta. This may be regarded as the 

 first well- determined appearance of the Pelecypoda in any region. 



Gasteropoda. — None known in the whole of the series of forma- 

 tions below the Arenig, where four genera seem to be recognized, 

 both in South Wales (St. David's) and in the Stiper-stones area. 

 Pleurotomaria ttanvirnensis, Hicks, Euomplialus corndeusis, Sow., 

 and forms of Ophiteta and Ehajphistoma occur. 



Pteropoda. — The six horizons of the Lower Cambrian yield 4 

 genera, Cyrtotheca, Stenotheca, Theca, and Conidaria, and are repre- 

 sented through the six divisions by 11 species : but only 2 occur in 

 the Longmynd group of St. David's under analysis {Theca antiqua, 

 Hicks, and T. penultima, Salt.). The Menevian beds of the same 

 area have yielded 6 species, to be noted under that group of rocks, 

 and the Lower and Upper Tremadoc 9 species. During the Arenig 

 and Caradoc periods onty, have we to record so many pelagic Mol- 

 lusca ; 8 species occur in the Arenig, and 10 in the Caradoc. 



Heteropoea. — Bellerojolion is the only form that occurs in these 

 lowest rocks ; but no species has been recorded lower than the so- 

 called Middle Lingula-flags, where, as B. cambrensis, Belt, the genus 

 first appears, associated with Hymenocaris vermicauda and Cono- 

 coryphe in the Upper Ffestiniog rocks ; but four of the five known 

 species are Tremadoc, where, specifically, the genus becomes of im- 



