546 C. Lapworth — On Scottish Monog rap f idee. 



which appear to unite in themselves the characteristics of two 

 distinct Graptolitic families. In these anomalous species the coeno- 

 sarcal tuhe originates from a point on the lateral margin of the 

 sicula a little removed from its broader extremity, adheres to it by 

 the proximal portion of its dorsal wall, and is prolonged beyond it 

 in the same general direction, exactly as in every species of the 

 MonograptidcB whose development has been thoroughly investi- 

 gated. But, after the coenosarc has developed a few hydro thecge, it 

 suddenly subdivides, and forms two branches, which grow side 

 by side at a tolerably equal rate, adhere to each other throughout 

 their whole extent by their flattened dorsal walls, and give to the 

 distal portion of the polypary the identical structure and appearance 

 of that of Liplogr aphis. 



Guided more especially by the appearances successively presented 

 by the sicula and polypary in the earliest stages of development in 

 Cenograptus, Hall, and its allies, I formerly advocated the view that 

 the sicula in all the bilateral genera threw out two buds — one from 

 each of its opposite lateral margins ; and, by analogy, extended it to 

 include adherent forms like Diplograptus and Climacograptus. There 

 is, however, another theory ; which, though it may appear at first 

 sight totally irreconcilable with the successive phases of the budding 

 polypary in some forms of the Diplograpti, is far simpler, and in all 

 likelihood will prove to be the true one. According to this theory 

 the sicula in all the Graptoloidea throws off a single bud only, and 

 thus theoretically invariably originates a single coenosarcal tube. 

 In Tetragraptus and its allies,^ this first-formed coenosarcal tube 

 (pro-coenosarc) is divided immediately at its origin into two 

 branches, and again subdivided within a very short distance. 

 Between the proximal ends of the two pairs of primary branches 

 which commence at these subdivisions, there is thus formed a short 

 horizontal bar, never more than one-fourth of an inch in length, 

 totally barren of hydrothecae, and having the sicula in its centre. 

 This is the true funicle of Hall, and evidence has recently been 

 obtained which makes it certain that, contrary to the general belief 

 hitherto, this is the only non-polypiferous portion of the polypary in 

 any of these highly complex genera. Making exception of the 

 genus PTiyllograptus, Hall (which is morphologically a Tetragraptus 

 with coalescent branches), there is no secondary subdivision of the 

 non-polypiferous portion of the coenosarcal canal in any of the 

 remaining Graptoloidea. In Diplograptus, Pleurograptus, etc., it is 

 once divided immediately after origin. In Monograptus and its 

 allies it is not divided at all. 



Hence we may look upon Dimorphograptus either as a Diplo- 

 graptid in which the division of the coenosarc is deferred till several 

 hydrothecEe have been developed ; or as a Monograptid (like Cyrto- 

 graptus), whose branches, instead of being free, are wholly coalescent. 



The discovery of this genus lends additional support to the theory 

 that the Monograptidm are the direct descendants of the Diplograp- 

 tidcB, and not of any of the compound monoprionidian genera. The 

 ^ Clonograptm, HaU ; Clematograptus, flopk, ; Loganograptus, Hall, etc. 



