I.xxxii BRITISH (IRA OTOLITES. 



7. Glossograptidae, virgulae coalescent, central. Gen. : 23, Glossograptus ; 

 24, Retiograptus ; 25, Lasiograptus. 



8. Retiolitidae, virgulse separate, lateral. Gen.: 26, Clathrograptus ; 27, 

 THgonograptus ; 28, Retiolites. 



This classification is elaborated in a detailed Analytical Table. 



Development. — In this paper Lapworth points out for the first time that in all 

 the bilateral genera included in the families assigned to the Graptoloidea, the 

 Graptolite polypary first becomes visible as a small, pointed, triangular, or rather 

 dagger-like " germ," which he names the sicula (already recognised by Richter 

 in the case of Monograptus and Diplograptus only, and denominated by him the 

 " foot"). In the majority of bilateral examples studied a solid axis is developed in 

 the outer wall of the sicula extending along its entire length, and a small pro- 

 tuberance or " bud " makes its appearance usually in the neighbourhood of the 

 sicula, and becomes moulded into a theca : a similar bud is given off from 

 the opposite margin, and from these primordial buds the two main branches 

 of the polypary are evolved by a process of continuous gemination. 



But while the sicula appears always to have been present, the place of origin 

 of the primordial bud or buds is somewhat different in the different genera, and in 

 some forms there still remains a doubt whether the polypary is not a direct out- 

 growth of the major (Dichograptus, etc.) or of the minor (Monograptus) extremity 

 of the sicula. The sicula normally ceases to grow after the primordial buds have 

 been given off and may occasionally become imbedded, absorbed, or obsolete ; but 

 in the great majority of cases it permanently retains its shape and form. " It is 

 simply this persistent sicula which constitutes the axillary spine in Dicello- 

 graptus" the "radicular bar" in Goenograptus, and the "radicle" in Didymo- 

 graptus, Phyllograptus," etc. 



The sicular or dorsal angle of the two main branches of the bilateral Grapto- 

 lites is adopted by Lapworth as the " angle of divergence," and he points out that 

 it ranges throughout the complete circle. It may be 0°, in which case the brandies 

 grow parallel with each other distally along the line of the sicula and coalesce by 

 their dorsal surfaces either for the whole of their length (Diplograptus) or for a 

 portion of it only (Dicranograptus). It may be less than 180° (Dicellograptus) or 

 may exceed 180° (Didymograptus), or it may be as high as 300°, when the branches 

 again coalesce (Phyllograpius). 



Structure. — As respects the structure of the monoprionidian Graptolite 

 Lapworth asserts that "the common portion of the polypary preserved in relief 

 appears to be composed of the conjoined bases of the successive theca3. These 

 IukI from each other in a single linear series. The budding orifices remain per- 

 manently open and form together a continuous tube or canal of communication for 

 the conveyance of the common body." lie restricts the application of the term 

 theca to the " exterior and separable portion of the chamber — in other words, to 



