No. 373-] THE STUDY OF GRAPTOLITES. 3 
the periderm, the specimens were prepared for the microtome 
according to the methods used by zoologists. 
By the application of these preparative methods histological 
and morphological discoveries have been made. 
The histology of the graptolites has been especially advanced 
by the researches of Holm (90), Sollas (94), and Perner (94). 
The last two investigators demonstrated the presence of three 
different layers in the periderm of Monograptus; vzz., a stronger 
middle layer between two thinner ones. Wiman (95) verified 
Perner’s observations as to Monograptus priodon and discerned 
the two outer layers in Diplograptus. The middle layer in 
Diplograptus contains the growth-lines observed repeatedly 
before. The histology of the Retioloidea has been studied by 
the above-named geologists and by Tullberg (82) and Térnquist 
(90, 93). Holm found three layers; vzz., smooth epidermic and 
endermic layers, which inclose the latticed network of chiti- 
nous threads, from which this group derives its name. 
In the Dendroidea Wiman ('95) observed the two outer 
layers. 
` However interesting the discovery of the differentiation of 
the periderm of the graptolites is, especially on account of its 
bearing on the question as to the zoological affinities of this 
group, it is surpassed in importance by the knowledge which 
has been obtained as to the morphology and development of the 
graptolites. In reviewing the progress made in these directions 
we will separately regard the Graptoloidea, Retioloidea, and 
Dendroidea. 
As an understanding of the conformation of the rhabdosome 
rests with the knowledge of its growth from the sicula, it will 
be opportune to review first the fundamental results obtained 
by Wiman as to the growth of the initial part of the rhabdo- 
somes of Monograptus (93), Diplograptus ('93), and some other 
Graptoloidea. The sicula of these consists of a thin-walled 
“initial part,” which is prolonged into a process, the “virgula,” 
and of the “ apertural part,” which shows distinct growth-lines 
and a three-spined symmetrical aperture (Fig. 1, s). From the 
sicula sprouts a new individual, the first theca (Holm considers 
the apertural part of the sicula already as “first theca”’), which 
