GRAPTOLITES OF NEW YORK, PART 2 449 



constancy of its differential characters in all specimens I rom the same horizon 

 leave no doubt that we have a mature though dwarfed form before us. 



Family monograptidae Lapworth 

 monoqraptus Geinitz, em. Jaekel et Freeh 



While the genus Monograptus is so profusely represented in the 

 Siluric of Europe that not only a great number of species have been distin- 

 guished but its distribution has also been the means of the elaboration of a 

 most refined system of Siluric horizons, North America has, besides a form 

 in the arctic North, yielded but two species, one in the Clinton beds of New- 

 York and one in the Siluric of Maine, and both of these are hardly more 

 than mutations of the most common and best known European species, M . 

 priodon Bronn. We can, therefore, restrict ourselves 

 to a consideration of the group of Monograpti of which 

 M . priodon is the representative. 



The genus Monograptus, as originally conceived 

 (Lomatoceras Bronn, Monoprion Barrancle and Grapto- 

 lithus of some authors) has been derived through forms 

 of Dimorphograptus from Diplograptus and Climacog- 

 raptus, by the reduction of the biserial arrangement of 

 their thecae to a uniserial one. In correspondence to 423 424 



Fig, 423, 424 Copies of 



this origan of the o-enus the sicula occupies the most distal r?™°™'* &B ??l*°L^ °»X 



^> ij X Jj 1 cl JJ o U a C 1 C ^ a u S nil U 



ri 111 ii 1 • ..Monograps us rectus, 



point 01 the rhabdosome and the thecae grow in a reversed respectively 

 direction, climbing up along the nemacaulus as in Diplograptus and Clima- 

 cograptus. This position of the sicula and the growth .direction of the first 

 theca (first downward and then upward) are well shown in some of our 

 Clinton specimens [see fig. 428]. 



The form of the thecae was extremely diverse in the genus Mono- 

 graptus as originally conceived and has led to the distinction of several 

 genera. Those with the thecae of a Climacograptus have been united by 

 Freeh [1897, p. 621] under Monoclimacis ; and Jaekel had previously placed 

 the forms with hood or coverlike ventral margin of the aperture under 



