132 



COELENTERATA 



PHYLUM II 



Family 3. Retiolitidae Lapworth. 



Biserial Graptolitoidea with straight rhahdosomes, the latter characterised hy a 

 network of delicate chitinous tracery (reticula) which forms the outward covering of 

 the walls of the thecae. 



Retiolites Barr. (Fig. 209). Ehabdosome with periderm attenuated and 



supported on a meshwork of fibres. Thecae arranged biserially, their apertures 



A B opening outward. Two virgulae attached to 



opposite sides, in the median plane. Ordovician 

 and Silurian. Subgenera : Gladiograptus Hop- 

 kinson and Lapworth ; Gothograptus Freeh. 



Family 4. Dimorphograptidae Lapworth. 



Uni-biserial Graptolitoidea, in which the proximal 

 portion is uniserial, bearing thecae of the general 

 Monograptus type ; the distal portion is biserial ivith 

 thecae of the Diplograptid type. 



Dimorphograptiis Lapworth. Silurian. 



Family 5. Monograptidae Lapworth. 



Uniserial Graptolitoidea, with simple or compound, 

 straight or convex rhabdosome and thecae of varied 

 form. 



Monograptus Geinitz [Monoprion Barrande ; 

 Pomatograptus and Pristiograptus Jaekel) (Figs. 

 193, 210). Rhabdosomes with only a single 

 row of thecae, which are in contact, usually 

 overlapping, their apertures entire or contracted, 

 often directed downward. Form of the rhab- 

 dosome may be straight, curved or sometimes 

 spirally coiled. Silurian and Devonian. 

 1). Rhabdosome simple, spirally coiled ; common 

 parts of thecae more or less linear and widely 

 Silurian. The zonal distribution of species in 

 been worked out in detail by Eisel. 



Variously branching Monograptidae. Silurian. 



Fio. 209. 



Betiolites geinitzianus Barr. Silurian. 



A, Specimen from siliceous schists of 

 Feaguerolles, Calvados ; natural size. 



B, C, Polyparies from Motala, Sweden. 

 75, Cross-section. C, Lower end, en- 

 larged ; calcareous matter dissolved 

 out by acid, v, Zigzag-shaped virgula ; 

 v', Uod-lil<e virgula ; th, Conjoined 

 walls of hydrothecae ; s', Crossbars 

 connecting the virgulae ; o. Apertures 

 (after Holm). 



Bastrites Barr. (Fig. 21 

 canal very narrow ; distal 

 se})arated from one another 

 Thuringia and Saxony has 



Cyrtograptxis Carruthers 



Range and. Distribution of Graptolites. 



Graptolites are excellent index fossils of the older Paleozoic rocks, owing 

 to their limited vertical range, and wide geographical distribution. The 

 simpler forms, such as are derived by a succession of budding from a primary 

 sicula (Axonolipa), are especially characteristic of the uppermost Cambrian 

 and lower half of the Ordovician rocks. The group as a whole becomes extinct 

 at the close of the Silurian, except for a few stragglers in the Devonian and 

 Carboniferous. The occurrence of these organisms in rocks of the same age 

 in all parts of the world is explained by the fact that while some forms were 



