54 F. H. McLearn — Lower Ordovician 



Bed B contains an unusual assemblage of forms, linking it, 

 on the one hand, with the Tetragraptus zone, and, on the 

 other, with the Didymograptus Mfidus zone. Didymograp- 

 tus v-fr actus has only been reported from the Lake District 

 (Elles and "Wood 1901) and is there confined to the Dichograp- 

 tus bed of the Middle Skiddaw slates. Abundant Didymo- 

 graptus nitidus and D. extensus are also more abundant in 

 the Tetragraptus zone and its equivalents, although D. exten- 

 sus may pass into the D. Mfidus zone, as it does at Levis, 

 Quebec (Raymond). D. gracilis is common in and highly 

 characteristic of the lower bed of the D. Mfidus zone of the 

 Deepkill (Ruedemann). It is rare in the Middle Skiddaw 

 slates (horizon not indicated), and is also found in the Phyllo- 

 graptns shales of Skattungbyn in Dalarne, Sweden (Ruede- 

 mann). D. acutidens is rare in the D. Mfidus zone of the 

 Deepkill, but present (Elles and Wood 1901) in the same hori- 

 zon at St. Davids and in southern Shropshire in the Llanvirn 

 (= Upper Arenig). Evans (1906) reports it from the D. Mfi- 

 dus bed of the Llanvirn of West Csermarthenshire, Wales. 

 The form present at St. John differs a little from Lapworth's 

 species, but is a very close variety (vide infra). 



Didymograptus v-fractus, common D. nitidus and D. exten- 

 sus, and the absence of D. Mfidus all indicate affinity with 

 the Tetragraptus zone, and especially its equivalent in the 

 Lake District, the Dichograptus zone of the Middle Skiddaw 

 slates. D. gracilis links the fauna with that of the lower bed 

 of the D. Mfidus zone of the Deepkill. D. acutidens indicates 

 affinity with the same horizon at St. David's, southern Shrop- 

 shire, and western Caermarthenshire. 



Bed B, therefore, combines common Tetragraptus zone 

 forms, including one confined to it, with two forms that usually 

 occur in the next zone. It seems best, then, to regard the 

 fauna of Bed B as a transitional one between those of the Tetra- 

 graptus and the D. Mfidus beds. 



A study of Raymond's lists (1914) of the Levis, P. Q., zones 

 shows that a similar early appearance of D. Mfidus forms takes 

 place there. Zone C 1 underlies the zone with D. Mfidus and 

 apparently holds the time of the Tetragraptus zone. In addi- 

 tion to a few common Tetragraptus zone forms and some local 

 species, it contains a few kiuds which in other localities occur 

 at a higher horizon. Thus Didymograptus similis is highly 

 characteristic of the D. Mfidus zone of the Deepkill, and at the 

 same horizon in northwestern Europe is represented by closely 

 related or identical species, as Ruedemann has shown. Ptilo- 

 graptus geinitzianus in the Deepkill occurs in the D. Mfidus 

 zone, although very rare. A species, D. cf. indentus, near to 

 or identical with Didymograptus indentus, is also found at a 

 higher horizon in Europe (Elles and Wood 1901). 



