60 Dr. H. A. Nicholson on the Graptolitidte. 



never be preserved in a fossil condition. With regard to 

 Dendrograpsus (PL III. figs. 16, 17) and Callograjysus (two 

 genera which more closely resemble the Sertularidte than any 

 other), the probabilities are, perhaps, in favour of their having 

 been fixed, though there is no decided evidence in support of 

 this view ; and the same may be said of Dictyonema. 



Allied forms. — The affinities of Graptolites as regards other 

 extinct organisms are few and uncertain. There exists, how- 

 ever, one allied form (PL III. fig. 19), Avhich I described last 

 year from the Dumfriesshire Shales under the name of Cory- 

 noides calicularis. In this the poly|ndom is in the form of a 

 simple elongated tube, without any central axis, furnished at 

 the base with two spines, and expanding distally into a toothed 

 cup or " hydrotheca." In general form Corynoides closely 

 resembles some of the Corynid® or Tubularidte ; but the " hy- 

 drosoma has certainly been free, and was never fixed by a 

 hydi-orhiza." Whether Corynoides should be included amongst 

 the Graptolitid*, or should be regarded as the type of a new 

 order, is doubtful ; but it is certainly allied to the Graptolites, 

 and greatly strengthens the belief that the latter belong to the 

 Hydrozoa. 



Conclusion. — I have now endeavoured to show that the 

 position of the Graptolitidse amongst the Hydrozoa is sup- 

 ported by the phenomena observed in their morphology, deve- 

 lopment, and reproduction, in their mode of existence, and by 

 the determination of allied forms. As to their exact place, it 

 is certain that they cannot be referred to any existing order 

 or even subclass of the Hydrozoa, and it is probable that they 

 stand in the same relation to the recent Hydrozoa that the 

 Trilobites do to the Crustacea. In the arrangement of their 

 parts and in their mode of growth, as well as in the nature of 

 their structural elements, they more or less resemble the 

 Hydroid polypes ; but they are widely separated by their free 

 hydrosoma. On the other hand, they approximate to the 

 oceanic Hydrozoa in the fact that they were free-floating 

 organisms, and in the possession, by some forms, of an organ 

 resembling a " float." In the present state of our knowledge 

 it seems, therefore, most advisable that the Graptolitidee should 

 be held to constitute a new subclass, which will hold an inter- 

 mediate position between the fixed and oceanic Hydrozoa, and 

 which might possibly, on the derivative theory of develop- 

 ment, be considered the primitive stock from which the 

 above existing sections of our living Hydrozoa have originally 

 diverged. 



EXPLANATION OF PLATE III. 

 Fig. 1. Graptolites sayittarius, Linn., nat. size : showing the radicle (n). 



