. Bulletin of the British Museum (Natural History), Geology. 174 P. LEGRAND only so far definitely recorded from the Silurian, would be the same as that in Kazakhstan (Oysu River section), (iii) Finally, rare specimens of ?G. (Glyptograptus) aff. persculptus have been gathered just below the top of the middle member of the Oued In Djerane Formation. The objections to the hypothesis are the following: (i) G. (Glyptograptus) sahariensis is very close to G. (Glyptograptus) tariti and has the aspect of a Silurian Glyptograptus. (ii) Diplograptus africanus seems to belong to the Coronograptus cyphu
. Bulletin of the British Museum (Natural History), Geology. 174 P. LEGRAND only so far definitely recorded from the Silurian, would be the same as that in Kazakhstan (Oysu River section), (iii) Finally, rare specimens of ?G. (Glyptograptus) aff. persculptus have been gathered just below the top of the middle member of the Oued In Djerane Formation. The objections to the hypothesis are the following: (i) G. (Glyptograptus) sahariensis is very close to G. (Glyptograptus) tariti and has the aspect of a Silurian Glyptograptus. (ii) Diplograptus africanus seems to belong to the Coronograptus cyphus Zone (Legrand 1976), and consequently there is a very small thickness for the Parakidograptus acuminatus Zone and the Cystograptus vesiculosus Zone. The sandstones that form the top of the middle member may be thought to be the equivalent of the zone. (iii) Parakidograptus acuminatus has not yet been found; one can think of the sandstones that form the top of the middle member as the equivalent of the biozone characterized by this species. However, nor has it been found near the Libyan boundary, where the shales take the place of the sandstones owing to the later transgression there, and where the sedimentation seems to have been more continuous. (iv) Perhaps in this apparently very confined area the vertical range of species many not have been absolutely the same as in less restricted regions. To conclude, two hypotheses can be proposed for the position of the Ordovician-Silurian boundary, but the highest seems the most likely. Moreover, there is no characteristic fauna of the Ordovician in the lower part of the section and this sets problems of correlation with the standard sections (Dob's Linn, Kolyma River, Yangtse Valley), and consequently this section in Algeria can only be a local reference. On the other hand, it has important palaeogeographical significance since it shows the beginning of the transgression onto the southeastern part of the Saharan shield before the en
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