THE AKENIG AND LLANDEILO E0CXS OF ST. DAVID'S. 641 



being finally brought round so that, instead of their dorsal margins 

 coalescing as in Diplograptus, their ventral margins actually face 

 each other in parallel lines. 



2. Appendages of the polypary. — The so-called " ornaments " or 

 extraneous appendages of the polypary in the Ehabdojjhora consist 

 originally of marginal spurs or processes, simple or branching, free 

 or anastomosing. They are either lateral, ventral, or terminal in 

 position. 



The lateral or peridermal appendages are strong spurs (Glosso- 

 graptus), or long branching processes (Neurograptus &c), originating 

 directly from the virgula, and forming when complete a longitudinal 

 series extending down the median (septal) line of the periderm at 

 right angles with the hydrothecce. 



The ventral or thecal appendages are either proximal, mesial, or 

 apertural in position. 



The proximal and mesial appendages are invariably azygous, simple 

 spurs, or spines, given off from a fixed point in the middle line of 

 the outer margin of the theca — the former at its proximal end 

 (Climacograptus), the latter midway along its length (Duello- 

 graptus). 



The apertural spines proceed from the outer margin of the orifice 

 of the hydrotheca, and either occur singly (Olyptograptus) or in 

 pairs (Orthograptus). Normally simple and free, they sometimes 

 support between them a flat chitinous plate or " vesicle." In some 

 genera (Lasiograptus &c.) they anastomose and form a continuous 

 braiding of marginal meshes. 



The proximal (or radicular) spurs, or first-formed thecal spines, 

 whether mesial or apertural, attain in many species an extraordinary 

 development ; and, like their diminutive homologues, they sometimes 

 support between them a " vesicle " or " disk f but they are not yet 

 known to branch or anastomose. 



3. Radicle. — This term was first suggested to Professor Hall by 

 his theory that some of the Ehabdophora were at least temporarily 

 affixed to the sea-bottom or to foreign objects. In the forms which 

 he believed to be permanently free, he clearly characterized it as 

 being actually the proximal prolongation of the solid axis {virgula). 

 In spite, however, of this rigid definition, the term has been applied 

 indiscriminately, by its eminent founder and by those palaeontologists 

 who have since employed it, both to the proximal end of the virgula 

 and also to the persistent sicula itself. A whole family (Dicho- 

 graptidse) of the Graptoloidea, and many genera belonging to other 

 families, have consequently been always figured in an inverted posi- 

 tion. Our improved acquaintance with the true relations of the 

 sicula and branches will effectually prevent the repetition of this 

 formerly natural, and perhaps inevitable error. The term radicle 

 may therefore for the present be retained with advantage exactly in 

 the letter of Hall's definition — i. e. as the distinctive title of the 

 proximal prolongation of the virgula, whether that of the polypary 

 itself (Diplograptidae), or that of the persistent sicula (Nema- 

 graptidye, &c). 



2u2 



