306 BRITISH GRAPTOLITES. 



(parietal, Holm) ; and the lino of contact along the sutural groove with the 

 opposing thecas of the alternating series (dorsal lists). 



As regards the relations of the lists to the polypary considered as a whole, it 

 naturally follows that when all the individual thecas are outlined as above, the thecal 

 lists constitute in combination a collective net-like or cage-like framework (clathria, 

 from Lat. clathri or clathra), which forms, as it were, a skeleton of the complete 

 polypary. Those thecal lists which then lie in the same general longitudinal line 

 combine in appearance or reality into a single longitudinal list or strand (Wiman) 

 running from end to end of the polypary, and the thecal lists which lie transverse 

 to these form connecting cross-bars or ledges (Gurley). 



Thus the dorsal lists combine into two longitudinal medial or septal strands, one 

 (which is usually zig-zag) on the obverse, and one (which is usually straight) on 

 the reverse aspect of the polypary. The pleural and other ventral lists combine into 

 four longitudinal ventral strands, which are generally somewhat undulating and 

 irregular. These upright strands are united in pairs by the cross-bars which are 

 formed by the apertural and parietal lists (somewhat as the sides of a ladder are 

 united by its rungs, or the sides of a lattice by its cross-bars). 



The skeleton or clathria attains its most perfect development in the family of 

 the Retiolitidas, in the typical species of which — lletiolites Qeinitzianus — its essential 

 elements have been made known to us by the researches, descriptions, and figures 

 of Tullberg, Tornquist, Holm, and Wiman (see Figs. 220 d—f). 



The longitudinal strands enable us conventionally to regard the structural 

 meshwork of the clathria as divisible into six fields (Wiman), scalars, or lattices: 

 namely, two ventral /attires, of each of which the sides are the two upright ventral 

 strands, and the cross-bars (which are normally horizontal) are usually the 

 apertural lists of the thecae ; and four parietal lattices, of each of which the 

 more or less upright sides are formed by one of the septal strands and one 

 of the ventral strands lying on the same aspect, and the cross-bars (which are 

 usually inclined) are the parietal lists. In addition to the cross-bars belonging to 

 these six lattices, the clathria is sometimes provided with tAVO sets of internal 

 transverse cross-bars or struts, formed of the aboral lists. 



Owing to the differences in the original shape of the polypary and thecas, and 

 the special parts of these which have been strengthened or differentiated as 

 lists or filaments, there are great variations in the extent of development and in 

 the details of the apparent skeleton as presented by the several species of Glosso- 

 graptidae and Retiolitidas. Some of the so-called strands are often only such in 

 appearance, being formed of different elements in different parts of their length ; 

 and the cross-bars of the apparent ventral lattice are not always the same in origin. 

 Tlic combined appearances presented in compressed specimens of the Glosso- 

 graptidae by (a) the clathria when partially developed, (b) the remainder of the 

 polypary, and (c) the various external appendages, are always difficult of interpre- 



