236 Mr. J. Hopkinson on a new Dendroid Graptolite. 



forms, we have some evidence indicative of a different mode 

 of existence. The stems of Dendrograptus are enlarged towards 

 their base, and sometimes present a sudden expansion or bulb, 

 which I have inferred may be the base or root, once attached 

 to another substance, or imbedded in the mud or sand of the 



sea-bottom." ''In those which I have termed Cal- 



lograjjtus^ the bases of the fronds are imperfect, but indicate, 

 according to analogy, a radicle or point of attachment like Den- 

 drograptus. In the more nearly entire forms of Dictyonema 

 known, we have not been able to observe the base ; but, from 

 their similarity in form and mode of growth to Fenestella and 

 Retepora, we have inferred their attachment either to the sea- 

 bottom or to foreign bodies." (20th Rep. New-York State Cab. 

 Nat. Hist., p. 238, ed. 1870.) 



The bearing of this on the question of the systematic posi- 

 tion of the Dendroidea alone remains for consideration. It 

 has already been shown that the Rhabdophora differ from 

 our recent Sertularian Hydroida only in their possession of a 

 slender rod or virgula, and in their having apparently been free. 

 The Dendroidea offer no such points of difference, being essen- 

 tially similar to the recent Sertularian zoophytes in their mode 

 of growth, as well as in their general form, and, as far as their 

 imperfect state of preservation enables us to determine, in their 

 intimate structure also. On this last and most important 

 point, however, we have no certain knowledge : while we 

 know of no characters whereby the Dendroidea can be sepa- 

 rated from the Hydrozoa, we are equally destitute of decisive 

 evidence of their structural difference from the Polyzoa ; nor 

 can we wonder at this when we consider how long these two 

 classes were grouped together under the general term of Zoo- 

 phyte or Coralline. 



Dictyonema certainly seems more Polyzoan than Hydrozoan 

 in its affinities, while Ptilogroptus^ on the other hand, seems 

 to be far more nearly related to the Hydrozoa than to the 

 Polyzoa ; and analogy with the true graptolites, which are 

 certainly Hydroids, would lead us to infer that the Dendroidea 

 have the same internal structure as they have. If this should 

 prove to be the case, the genera Ptilograptus and Dendrograp- 

 tus would fall naturally into families already existing in the 

 sub-order Thecaphora (or Sertularina) , while for Callograptus 

 and Dictyonema^ which have their branches more or less re- 

 gularly connected together by transverse processes, a new family 

 would have to be instituted. At present we do not know of 

 any tangible character whereby the Dendroidea, considered as 

 a single group, can be separated as a distinct sub-order from 

 the Thecaphora. In the mean time the term Graptolite may 



