54 Traihsactions. 



Llandovery. It will be observed that it shows no row of denticles 

 or hydrothecse as in Moiiogi-apUis Sagittarius, Linn, M. Nilssoni, 

 Barr, and many others, but merely elliptical apertures forming 

 the cell's mouths on both sides of a cylindrical frond or tube, 

 which are not opposite to each other, but alternate. Its width 

 when fully developed is about one-eighth of an inch. The solid 

 axis is prolonged both proximally and distally to a considerable 

 length. The external appearance of the frond varies according 

 to the amount of pressure it may have sustained. There are 

 about 30 hydrotheciB to an inch. The cylinder or frond of 

 C'iimacograptus teretiuscuhcs (His. ) is made up of two semi-circular 

 compartments, placed back to back with the septum, and the 

 solid axis placed in the centre forming a mutual gable or pai-tition, 

 as it were, between the two colonies of Zooids, having each their 

 own row of cells and common canal, each colony being quite 

 independent of the other. Localities for this species are Dobb's 

 Linn, Hartfell, Beld Craig, &c. 



Genera DidymogrcqAus. — The polypery is composed of two 

 simple branches springing bilaterally from the radical or initial 

 point. The branches are monopriodon, assuming difterent angles 

 in different species. The cells, or " hydrothecse," are generally 

 opposite to the radical or initial point, but in some examples they 

 are found on the same side. The size of the radical or spine 

 varies very much in different species, and in some it is altogether 

 awanting. It is not easily determined to what uses these spines 

 could have been applied, as there does not seem to be 

 any evidence that they had been objects of attachment. If we 

 look at their different positions with the spines inside of the 

 angles or concave, one would think it scarcely possible for them 

 to be used for such a purpose, more especially as some of them 

 are without any radical or initial point whatever. The genus 

 Didyviograptus was originally founded by M'Coy (1851), and 

 takes in those Graptolites which ai-e " bifid," or of a twin 

 character. The vertical range of this genus lias its commence- 

 ment in the Skiddaw and Quebec gi'oups, and attains its maximum 

 in those rocks in which it is represented by numerous and remark- 

 able examples. It likewise occurs in the lower Llandeilo, and is 

 also well represented in the upper Llandeilo rocks, but is scarcely 

 known in the Caradoc period of this countiy. Several examples 

 are got in the rocks of this age in America, but not one is known in 

 the upper Silurian, and it may safely be said to be characteristic 

 of the upper and lower Silurian periods. 



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