544- MOW YOHK STATE MT'SEfM 



selves ai'ound Dendrograptus and Dictyonema, which previously were only 

 Avith some doubt united with the graptolites ; and, further, that among the 

 latter the forms Avith one series of thecae were opposed to those with two 

 or more series of thecae. The arborescent forms were separated as Cladoplwra 

 (Hopkinson) — now currently designated Dendroidea (Nicholson) — from the 

 Wiabdophora, a term proposed by Allman under the wrong supposition 

 that all gi'aptolites proper had a strengthening rod, the virgula. 



The number of series of thecae was first used systematically for the 

 erection of several divisions ])y Hopkinson, Avho pi'oposed the groups Mono- 

 prionidae, Mono-Diprionidae, Diprionidae and Tetraprionidae. Hopkinson's 

 suggestions have been further elaborated in Lapworth's complete system 

 (1873). In this the Rhabdophora are divided into two sections, viz (1) 

 Graptolitidae and (2) Betioloidea. The Graptolitidae are characterized as 

 developing the rhabdosome from a true sicula ; having a coenosarc, from which 

 originates a single series of thecae only (the Diprionidae included in this 

 section were considered by Hall and others as possessing a "dibrachiate 

 monoprionidian polypary, whose branches coalesce by the Avhole of their 

 dorsal surfaces") and as having a dorsal virgula embedded in a narrow- 

 groove on the exterior surface of the periderm. The Retioloidea, on the 

 other hand, were stated to have their rhabdosome never developed from a 

 siculiform " germ," as having a coenosarc originating a double series of thecae, 

 and as possessing an epiderm which is more or less supported by a framework 

 of chitinous filaments. 



The first section w^as further divided into (1) Monoprionidae, compris- 

 ing the families Monograptidae, Nemagraptidae (Leptograptus, Amphigraptus, 

 Pleurograptus, Nemagraptus, Coenograptus), Dichograptidae (Didymograptus, 

 Tetragraptus, Dichograptus, Loganograptus, Clonograptus and four unnamed 

 genera, represented respectively by Gr. multifasciatus, milesi, 

 V a g a n s and richardsoni); (2) the Mono-Diprionidae, comprising the 

 family Dicranograptidae, with the genera Dicellograptus and Dicranograptus ; 

 (3) the Diprionidae, comprising the family Diplograptidae (genera Climaco- 



