GRAPTOLITES OF NEW YORK. PART 1 539 



In some genera of both series, the Diplograptidae and Climacograptidae, 

 the peridermal walls of the thecae may become dissolved into a system of 

 meshes. In the genus Retiograptus, which is, in the faunas here described, 

 represented by one species, this dissolution is incomplete, and the meshes 

 appear only along the apertural margins. This genus appears also to stand 

 apart from the other Axonophora by the vertical position of the thecae on the 

 axis. Owing to its being known only from shale material it is, however, but 

 little understood, and its characters need further elucidation. 



The same can be said of the genus Trigonograptus, to which a character- 

 istic species of the zone with Dipl. dentatus has been referred. The most 

 striking and apparently sole differential character of this genus is the lack of 

 any interruption of the outer margin by apertures. This is, as our material 

 shows [pl.l7], produced by the peculiar position of the apertures, which lie 

 so oblique to the axis of the thecae that they are subparallel to the principal 

 axis of the rhabdosome. 



Certain Diplograptidae are characterized by long apertural spines. These 

 attain their extreme development in the species referred to Glossograptus. 



In the Dendroidea and axonolipous Graptoloidea only persons of the first 

 order (thecae) and of the second order (rhabdosomes) have been observed ; 

 in some of the Axonophora, however, (Diplograptus, Retiograptus) it has 

 been found that the rhabdosomes, each of which originates from a sicula, 

 combine into a colony which is thus a person of the third order. 



On receptacles for organs of sexual reproduction, see chapter on mode of 

 reproduction, page 519. 



9 Histology and chemical composition of the periderm 



Richter [1871] Avas the first to call attention to the structure of the 

 peridenn. He discerned two layers, an exterior, very flexible one, which is 

 again composed of two thin lamellae, and a thicker internal one, which is pro- 

 vided with transversal ridges. 



Glimbel treated, a few years later, polished specimens ^vith acids and 

 came to a like conclusion with Richter. He also inferred that the 



