Distribution of the Rhabdophora. 361 



the inconstant pseudo-genera that complete its special division 

 of the family. 



The numerous Diplograptidas present are generally of long 

 range ; but Climacograptus Scharenbergi^ Lapw., Diplograptus 

 euglyphus, Lapw., D. rugosus, Emmons, and D. perexcavatus } 

 Lapw., belong to the lower zones only, and D. quadrimucro- 

 natus, Hall, and Climacograptus tubuliferus, Lapw., are re- 

 stricted to the upper beds. 



Of the Lasiograptidas we here seem to meet with the last 

 survivors. Lasiograptus Harknessi is found in the lower 

 strata, and L. margaritatus, Lapw., in the middle. The 

 former is accompanied by the latest known forms of Glosso- 

 graptus. Of new forms the most remarkable are the fore- 

 runners of the true Retiolites with its superficial network, 

 here represented by the strange forms Retiolites Jibratus, 

 Lapw., and R. ? eucharis of Hall. 



Upper Caradoc (Hartfell). — In the Moffat area these strata 

 are comparatively barren, and the known fossils are restricted 

 as yet to two zones, one near the base of their subformation, 

 the other at its summit. The former, which is seen as a 

 narrow black seam in the " Barren Mudstones " of the shales, 

 is crowded with a peculiar species of Dicellograptus (D. com- 

 planatus, Lapw.) and a few forms of Diplograptidas. Scanty 

 as are the fossils of this zone, its beds are easily identified 

 thereby in Girvan, in Ireland, and even in Scandinavia, 

 where they aid us in fixing the Trinucleus-shalQS of Southern 

 Sweden as being of true Upper Bala age. 



The second zone, that of Dicellograptus anceps, has even a 

 scantier fauna, consisting as yet merely of the forms D. anceps, 

 Nich., Diplograptus truncatus, Lapw., Climacograptus bicor- 

 ntSj Hall, and 0. scalaris, His.,var. ; which, however, reappear 

 unmodified in the Drummuck beds at the summit of the Bala 

 of Girvan, and are also present in the Triniccleus-be&s of 

 Sweden. 



The mortality in families, genera, and species of Rhabdo- 

 phora. in the Upper Caradoc beds is extraordinary. The 

 entire families of the Dicranograptida?, Leptograptidas, and 

 Lasiograptidse disappear from sight altogether. The only 

 families that survive into the Llandovery are those of the 

 Diplograptidse and Retiolitida?, and these only in a very 

 degenerate form. With the exception of a doubtful variety 

 of the conventional species Climacograptus scalarisofHis'mger, 

 not a single form found in the Bala rocks has hitherto been 

 met with in strata of Llandovery age ; so that, as far as the 

 Rhabdophora are concerned, the palaaontological break between 

 the Ordovician and Silurian systems seems to be complete. 



