8 Belt — On the Fauna of the " Lingula Flags." 



consideration. Tliey come in like an invading host, and the few 

 species of Olenidce and AgnostidcB that struggle on upwards are out- 

 numbered, and, as it were, crowded out, by the intruders. It is an 

 interesting inquiry — from whence did the latter come? but our 

 materials are too scanty to furnish a reply. It is, however, to be 

 noted, that at least two of the Lower Cambrian genera which are 

 entirely absent throughout the Maentwrog, Festiniog, and Dolgelly 

 groups are represented in the Tremadoc and Arenig strata by two 

 families, greatly modified it is true, but yet showing many points 

 of structural affinity with the much more ancient genera. Micro- 

 discus, so abundant in the Menevian beds, comes back to us after a 

 long absence in the Trinucleidce of the Tremadoc and Arenig groups ; 

 and the Menevian Conocoryphe are represented by the CalymenidcB of 

 the Arenig and higher rocks, and are likewise absent from the inter- 

 mediate strata. 



I think that it is not improbable that in the Tremadoc epoch we 

 behold the return of a fauna driven from our area at the close of the 

 Lower Cambrian period, and which has, in the meantime, been greatly 

 developed, so far as the Trilobita are concerned, in some other area. 

 This supposition would be a hazardous one to propound on the 

 evidence of the Trilobites alone, but it is rendered more feasible by 

 a study of the lower forms of life that accompany them. The sea. 

 towards the close of the Lower Cambrian period must have teemed 

 with life. Besides the various genera of Trilobites, Pteropodous 

 Thecce must have swarmed, along with species of lAngulella, Obolella, 

 Discina, and Protospongia. When the Trilobites disappear, these 

 lower organisms disappear also. A few specimens of Lingulella 

 have, it is true, been found in the Maentwrog beds, but they 

 belong to a different type from the Lower Cambrian species, and 

 Theca, OboleJla, Discina, and Protospongia are unknown. In the 

 Festiniog beds, one, and perhaps two, species of Lingulella abound, 

 but they are of the same type as the Maentwrog species, and 

 the other genera are still absent. In the Dolgelly beds we have 

 the rare occurrence of Obolella, and a single specimen of Proto- 

 spongia has been found in a loose stone, believed to be from these 

 strata. The Lingulella are still of a different type from the Lower 

 Cambrian forms, and not a trace of Theca nor Discina have occurred. 

 When, however, we pass into the Lower Tremadoc strata, all the 

 lower types of life present in the Lower Cambrian rocks come back 

 to our area, almost unchanged specifically. Thecce resembling the 

 Lower Cambrian forms again abound, along with species of 

 Obolella, Lingulella, Discina, and Protospongia, so like the Lower 

 Cambrian species that I doubt whether they can be distinguished 

 from them. When I first found this Lower Tremadoc fauna upon 

 the flanks of Mynydd Gader, near Dolgelly, I doubted whether 

 I had not come upon Menevian strata, brought in by some great 

 fault, so similar were the lower forms of life ; and it was only the 

 presence of Niobe Homfrayi, and Asaphus innotatus, that assured me 

 that the rocks were really of Lower Tremadoc age. Lingulella ferru- 

 ginea, Obolella maculata, Discina labiosa, Protospongia fenestrata, and 



