502 C. Lapivorth — Oil British Graptolites. 



central branch, and examples of Diplograptus afford evidence that it 

 niajr possibly grow even more rapidly than the branches, and give 

 origin to that terminal " vesicle " which forms so extraordinary an 

 appendage in a few species. 



I have not yet been able to work out the development of the 

 unilateral Graptolites (Monograptidce) so fully in consequence of the 

 generally minute size of the sicula. I am unable to say whether the 

 polypary in these species extends along the sicula as in Diplograptus, 

 or whether it does not simply originate from its minor extremity, in 

 the manner of the central branch in Leptograptus. Should the latter 

 prove to be the true method of development, this circumstance, read 

 in conjunction with the facts to be presently noticed in connexion 

 with the vertical range of the Graptolites, enables us to summarise 

 the foregoing conclusions under the following general rule. 



In the earliest siculate Graptolites known (Dicliograptus, etc.), the 

 polypary originates from a dichotomising outgrowth from the major 

 extremity of the sicula; in the second type {Diplograptus, etc.), it is 

 formed from two opposite buds thrown off from the lateral margins 

 in the immediate neighbourhood of the major extremity ; in the 

 third type {Coenograptus, etc.) the two buds arise from points equi- 

 distant from both extremities; in the fourth stage {Leptograptus, etc.), 

 in addition to the two lateral branches, a third branch is formed by 

 the continued outgrowth of the minor extremity of the sicula ; and 

 finally in the last and most modern type {Monograptus, etc.), the 

 two lateral buds are apparently wanting, and the polypary is 

 developed from the minor extremity alone. 



The " angle of divergence " (sicular or dorsal angle) of the two 

 main branches of the bilateral Graptolites, varies throughout the 

 complete circle. It may be (F, in which case the branches grow 

 parallel with each other distally along the line of the sicula, and 

 coalesce by their dorsal sui'faces, either for the whole of their length 

 {Diplograptus), or for a portion of it only (Dicranograptus). It may 

 be less than 180° {Dicellograptus) ; it may exceed 180° {Dickjmo- 

 grapius) ; or it may be as high as 360°, when the branches again 

 coalesce {Phyllograptus) . When the sicular angle is less than 180°, 

 the proximal end of the complete polypaiy is formed by the major 

 extremity of the sicula ; and when it exceeds 180°, it is formed by its 

 minor extremity. 



STKUCTURE. 



The common portion of a polypary of Monograptus preserved in 

 relief appears to be composed simply of the conjoined bases of the 

 successive thecae (chambers). These bud from each other in a single 

 linear series. The budding orifices remain permanently open, and 

 form together a continuous longitudinal tube or canal of communica- 

 tion for the conveyance of the "common body. The line of junction 

 of these chambers can be traced on all examples preserved in the 

 round, completely to the virgula, which lies imbedded in a narrow 

 groove impressed on the dorsal walls of the chambers. The line of 

 junction is naturally thickened, and projects into the common canal 

 as a rounded shelf or flange. It will obviously be found convenient 



