86 NEW YORK STATE .MUSEUM 



cronatus and Glossograptus w h i t f i e 1 d i are here shown to 

 possess a like composition. This leaves in the division of Diplograptus 

 with spines but Diplograptus aculeatus Lapworth and D. insec- 

 tiformis Nicholson. 1 



In regard to D. (Idiograptus) aculeatus, Lapworth [1880, 

 p. 171] makes the following significant remarks: "The test is of great 

 tenuity, and is frequently invisible ; an outline only of the angles of the 

 fossil is preserved in strong chitinous threads. The zigzag septal line seems 

 to be formed of a single thread of this nature, apparently of no greater 

 thickness than that which outlines the hydrothecae." D. insectiformis 

 is a very small form and therefore not favorable for the observation of the 

 discussed structure, especially if the latter is but little developed and the 

 outer layer rather strong. 



In surveying the evidence, given by the species here cited, we feel that 

 we can venture the general proposition that the spinose forms of "Diplo- 

 graptus" possess as a rule a layer of retioloid mcsJics and as a corollary that 

 the development of this layer of meshes and ledges is roughly proportional to 

 that of the spines. 



From this proposition it can be deduced that the spines are casu- 

 ally connected with the principal parts of the retioloid layer, i. e. with the 

 main ledges, and secondarily also with the entire meshwork. It is to be 

 assumed that the formation of spines required basal braces or supports 

 which grew out like roots from the bases of the spines into the surrounding 

 peridermal tissue, and formed the ledges by uniting for mutual support. 

 It is in this connection to be noted that the spines in the species cited not 

 only proceed from the main ledges, but also lie in the points of intersection 

 of the ledges. The formation of the main ledges would easily induce that 

 of further connecting bars and thus lead to a retioloid structure, especially 

 since the combined formation of the spines and of the layer of meshes will 



1 1) . u p Ian die us Wiman, which is cited by Freeh among the spinose diplograptids, 

 does not properly belong there, since it only possesses the lateral spines of the first two 

 thecae, like nearly all other diplograptids and < limacograptids. 



