132 



COELENTERATA 



PHYLUM II 



Family 3. Retiolitidae Lapworth. 



Biserial Graptolitoidea with straight rhahdosomes, the latter characterised hy a 

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

 the walls of the thecae. 



RetioUtes Barr. (Fig. 209). Rhabdosome with periderm attenuated and 

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



opening outward. Two virgulae attached to 

 opposite sides, in the median plane. Ordovician 

 and Silurian. Subgenera : Gladiograptus Hop- 

 kinson and Lapworth ; Gothograptus Frech. 



Family 4. Dimorphograptidae Lapworth. 



Uni-biserial Graptolitoidea, in which the proximal 

 portion is uniserial, hearing thecae of the general 

 Monograptus type ; the distal portion is biserial with 

 thecae of the Diplograptid type. 



Dimorphograptus Lapworth. Silurian. 



Family 5. Monograptidae Lapworth. 



Uniserial Graptolitoidea, with simple or Compound, 

 straight or convex rhabdosome and thecae of varied 

 form. 



^^^- 20^- Monograptus Geinitz {Monoprion Barrande ; 



RetioiitesgeinUzianusB.vv. mun^n^ 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- 



A, Specimen from siliceous 

 Feuguerolles, Calvados ; natural size 



B, C, Polyparies from Motala, Sweden. 

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

 larged ; calcareous matter dissolved 

 out by acid. v, Zigzag-shaped virgula ; 

 ■y', Rod-like virgula ; th, Conjoined 

 walls of hydrothecae ; s', Crossbars 



connecting the virgulae ; o, Apertures dosome may be Straight, curved or somctimes 



spirally coiled. Silurian and Devonian. 



Bastrites Barr. (Fig. 211). Rhabdosome simple, spirally coiled; common 

 canal very narrow ; distal parts of thecae more or less linear and widely 

 separated from one another. Silurian. The zonal distribution of species in 

 Thuringia and Saxony has been worked out in detail by Eisel. 



Cyrtograptus Carruthers. Variously branching Monograptidae. Silurian. 



Range and Distribution of Graptolites. 



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

 to their limited vertical ränge, 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 



