204 



CCELENTERATA. 



widely represented in recent seas, but they possess a chitinous poly- 

 pary which seems to be quite as well suited for preservation in fine- 

 grained sediments as is that of the Graptolites. It is therefore a 

 matter difficult of explanation that, except in late Quaternary deposits, 

 no trace of an absolutely undoubted Sertularian or Campanularian, 

 similar to the ordinary existing forms of the order, has been as yet 

 discovered. On the other hand, the ancient Palaeozic deposits have 

 yielded the remains of various extinct organisms, which have often 

 been referred to the Graptolitoidea, but which, with greater prob- 

 ability, may be regarded as early types of the Thecaphora. One of 

 the most interesting of these is the Cambrian and Ordovician genus 

 Dendrograptus, in which the organism had the form of a spreading 



36. — Dictyonema retiforme, Hall. Silurian (Nia- 

 gara Group), United States. (After Hall.) 



Fig. 87. — A branch of 

 Ptilograptus pluniosus, 

 Hall. Enlarged. Ordo- 

 vician (Quebec Group), 

 Canada. (After Hall.) 



and branched frond, furnished with a strong footstalk (fig. 85), by 

 which it was probably attached to some foreign body. The branchlets 

 carry upon one side a series of little chitinous cups or " hydrothecae " 

 (" cellules "), which agree with the similar structures of the Grapto- 

 lites in partially overlapping one another. 



In Dictyone7iia (fig. 86) we have organisms resembling Dendro- 

 graptus in many respects, but not possessing any footstalk. The 

 frond is branched and plant-like, and is fan-shaped or funnel-shaped 

 in form. It is not certainly known whether the organism was attached 

 by its base or not ; but there is the strongest probability in favour of 

 its having been fixed. The branches radiate from the base, running 

 nearly parallel with one another, and often bifurcating. They are 

 united to one another at short intervals by numerous, irregular, 



