GLOSSOGRAPTIMC. 305 



The anastomosing processes are somewhat stout at their origin, becoming more 

 filiform as they extend ; and in several species they unite exteriorly with the 

 corresponding processes above and below, and thus form in combination a more or 

 less continuous external meshwork or lace work (lacinia) completely surrounding 

 the theca-bearing portion of the polypary (Figs. 215 a and e). 



A second characteristic of the Glossograptidae which is intimately related to the 

 first (compare Ruedemann, ' Grapt. New York,' Part 2, pp. 69 — 87) is afforded by 

 the distinct strengthening of the edges and angles of the polypary and thecse, and the 

 simultaneous attenuation of the intervening parts of the test. 



Within the limits of the family of the Diplograptidae, as we have seen, there is 

 an occasional tendency for the test to become somewhat thickened along certain 

 definite lines. Thus, in Orthog. quadrimucronatus this thickening takes the form 

 of a well-marked selvage, band, or flange, surrounding the edge of the aperture; 

 while in Amplexog. perexcavatus not only is the apertural margin strengthened 

 in this way, but also the ventral angles of the test around the excavation. 

 But in the family of the Glossograptidae this tendency to local thickening and 

 strengthening becomes progressively developed and intensified. Not only may 

 the apertural margins and ventral angles of the thecse become strengthened in this 

 way, but the strengthening may be continued along the outer line of contact of the 

 thecal walls across the main body of the polypary, into and even along the median 

 sutural line itself, the course of which may become marked by a continuous 

 strengthening as definite as that marking the apertural margins. In all cases the 

 local strengthening seems to be attained at the expense of the remainder of the 

 test, which throughout the whole family of the Glossograptidae is remarkably thin. 

 As this progressive differentiation is followed through the various genera 

 and species, these narrow bands, selvages, or lists become more and more 

 rounded and cord-like, and may eventually present the appearance — especially in 

 examples preserved in pyrites — of wire-like fibres, coarse threads, or strong 

 filaments, which are strikingly contrasted with the attenuated parts of the test 

 which they support- 

 Within the limits of the related families of the Glossograptidae and the 

 Retiolitidae occur forms showing almost every gradation between those in which 

 only a few of the edges and angles are strengthened by lists or filaments, and those 

 in which all the edges and angles are outlined in this manner. 



These lists may be regarded from two distinct points of view — (a) as related to 

 the elemental structure or theca, and (b) as related to the compound structure or 

 polypary. 



As respects their relation to the individual theca, they may outline the lip of its 

 external opening [apertural or oral lists) ; the sides of that opening {pleural) ; the 

 angles of the excavation (mesial) ; the lip of its internal budding orifice (portal 

 or aboral) ; the outer line of contact with the theca immediately above or below 



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