334 BRITISH GRAPTOLITES. 



whole of the polypary with the exception of the thecal apertures. The tracery 

 of this reticula is sometimes filiform and subregular, but is often of irregular 

 breadth and outline. Its minute meshes vary in form from circular to polygonal 

 and quadrangular, but there is rarely any constancy in the direction of the 

 component threads. 



Broadly speaking, it may be inferred that this reticula represents a further 

 development of the tendency to the general attenuation of the test and the local 

 concentration of its material along more or less definite lines, resulting in a net-like 

 structure. In the family of the Glossograptidae this tendency has originated 

 the clathria, and in the present family it is carried to an extreme, so that the 

 whole of the visible test of the polypary has become reticulate. 



It may be regarded as certain, however, that the visible network of the test 

 in the Retiolitidce (namely, the reticula and clathria combined) does not represent 

 the entire thickness of the periderm. The researches of Richter, Wiman, Perner, 

 Sollas, etc., go to show that the periderm is composed of at least three layers [the 

 endochiton, the mesochiton, and the ectochiton], and that the network belongs 

 to the middle layer, being overspread by the outer layer and underlain by the 

 inner layer, both of which are membranous and continuous, but of such extreme 

 tenuity that they are very rarely discernible. 



The clathria also attains its fullest development in the family of the Retio- 

 litidas. That of the species Betiolites (Gladiograjrtus) Geinitzianus is especially 

 complete, and is fully representative of the original form and structure of the 

 polypary and thecae. In many species, however, the clathria, although probably 

 present as such, is remarkably inconspicuous, little of it being preserved beyond 

 the septal strands and parts of the parietal lists. 



A few forms afford evidence of the presence of definite interthecal jplanes or 

 floors. These are usually membranous, but are so thin as to be visible only in 

 rare cases. 



The polypary itself varies much in form within the limits of the family ; it may 

 be long or short, narrow or broad. In transverse section it varies from concavo- 

 convex to subquadrangular. 



The characters of its proximal end are rarely exhibited, but occasionally there 

 is evidence leading to the inference that the mode of development was similar to 

 that in D'vploijrapUix. In one British species (Fig. 226 a) a well-preserved sicula 

 is shown, embraced as in Diplograptus by the earlier thecas. 



The theca? in the Retiolitidse present a considerable amount of variation in the 

 different species. In some (Fig. 220 c) they are quadrangular tubes in contact 

 throughout, so that four walls or sides are denned : namely, two exterior or lateral 

 walls (one obverse and one reverse) and two interior Avails (a floor or basement 

 Avail and a roof or covering wall). In others (Figs. 220 b and c) the terminal 

 portion of the theca is free as in Glyptograptus, etc., and there appears to be a 



