THE GRAPTOLITES AND CORALS 289 



Each square is the distal portion of a theca, with its 

 aperture horizontal and opening upwards into the exca- 

 vation above it. The middle portion of each theca is 

 bent horizontally, and the proximal portion is again 

 vertical. 



Specimens of Clirnacograptus are usually flattened in 

 the plane which bisects both columns of thecae, and 

 thus give a profile view; but occasionally they are 

 flattened in the plane separating the two columns : they 



a b 



FIG. 86. CLIMACOGRAPTUS WILSONI, LAPWORTH, LOWER 

 CARADOCIAN. 



a, Initial part of polypary (X i), showing sicula with large vesicle, and six 

 thecae of each column ; b, distal part of polypary ( x i), showing form 

 of thecae and "excavations " clearly on the left side. (After Elles and 

 Wood.) 



then show one set of thecae only in full face, and from 

 its ladder-like appearance this is called the scalarijorm 

 view. 



3. Monograptus priodon (Fig. 87) is a very 

 abundant graptolite in the lower part of the Wenlock 

 Shale, though not restricted to that horizon. Its 

 polypary is unbranched and attains a considerable 

 length up to a foot (25 c.m.) and more. The sicula 

 lies on one side of the proximal end, and from its apex 

 arises a stout virgula which supports a single column of 



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