Mr. J. Hopkinson on a neio Dendroid Graptolite. 235 



have not the same zigzag direction, and the whole poljparj 

 is more diffuse and irregular in form. 



But the distinctive feature in the specimen of CaUograptus 

 radicans described above is its possession of a hydrorhiza, or 

 rather, I should perhaps say, the ])reservation of its hydro- 

 rhiza ; for the presence of this organ in a single specimen of 

 one sj)ecies should suffice to prove its former presence in all 

 — to show, in fact, that it is an essential organ of the genus 

 Gallogr aphis. 



From the imperfect manner in which these Silurian fossils 

 are usually preserved, we cannot wonder that a delicate organ, 

 whose function it was to attach to some other substance the 

 more durable portion of the organism of which it formed a 

 part, has not before been found in connexion with this portion. 

 None of these dendroid graptolites has yet been found 

 attached to any other body. Their proximal termination is 

 usually imperfect, and often has an irregular margin as if it had 

 been broken. Such fracture, when the polypary was severed 

 from the substance to which it was attached, would most easily 

 take place at the junction of the hydrocaulus with its hydro- 

 rhiza. 



In the rocks in which graptolites occur, other fossils are 

 seldom found ; but in the graptolite beds from which this 

 specimen was obtained a large Comdaria [C. Homfrayi) 

 abounds^ and in a thin zone in which this and other species of 

 Callograptus and Dendrograptus occur in profusion it is 

 especially abundant. Upon this Comdaria, which is some- 

 times covered with graptolites, and also upon other fossils 

 which are occasionally associated with it, some of these dendroid 

 forms may perhaps have grown ; but no connexion has jet 

 been clearly seen. 



We are not without evidence that the other genera of the 

 Dendroidea were similarly attached to foreign bodies or to the 

 sea-bottom. Even if this were wanting, these dendroid grap- 

 tolites are so nearly allied to each other [Callograptus forming 

 an intermediate link between Dictyonema and Dendrograptus^ 

 to which also Ptllograptus is nearly allied) that we might safely 

 have inferred that the mode of existence of all these forms was 

 the same. But the genus Dendrograptus has already furnished 

 evidence of the fixedness of these dendroid forms. Professor 

 James Hall, after expressing his belief that the true grapto- 

 lites " in their mature condition were free floating bodies in 

 Silurian seas," thus treats of the mode of existence of the den- 

 droid graptolites : — 



" In regard to another group, including Dendrograptus ^ 

 Callograptus, and Dictyonema, as well as one or two other 



18* 



