DISTRIBUTION IN TIME. 185 



This table will, of course, not be regarded as representing the aetual distribution in time of 

 the IIydkoida. It is only an expression of the prineipal facts in the present state of our 

 knowledge of liydroid pala.'ontology, and it will be at once seen how very few these are. From 

 by far the greater number of formations we have no evidence of the existence of hydroid 

 life, while, if we except the graptolites, the examples which have come to us from any formation 

 whatever are very few, the vast majority of hydroid families being as yet unable to claim any 

 fossil representative. This, however, we can scarcely hesitate to refer to the defects of the record 

 rather than to the absence of hydroid life in past epochs of the globe. ^ 



' xVfter the above pages on the palreontological distriliution of the Hydhoida Lad been printed I 

 became acquainted with 11. Richter's paper " Aus den Thiiriugischen Schiefergebirge," published in the 

 ' Zeitschrift der Deutschen Geologischen Gesellschaft/ xxiii Band, 1 Heft, 1871. 



The author here describes the graptolites of the Upper Silurian strata of Thuriugia, but the paper 

 is chiefly occupied with a dissertation on the structure and affinities of graptolites in general. It 

 contains some valuable original observations, and, as the views advanced differ in certain points from 

 those of other German and of the British and American observers, I have thought it necessary to refer 

 to them here. 



He describes the test as composed of two laminae, an inner thick one — usually marked by oblique 

 striae — and an outer thinner one, the latter being itself composed of two thin lamellae, which repeat 

 the striated condition of the former when such is present. If this be really the nature of the graptolite 

 test, the condition in which the Thuringian specimens have come down to us must be exceptionally 

 fitted for the detection of structure, as no evidence in favour of this view seems obtainable from any 

 British or American specimens. 



He describes as an " organ of attachment " or "foot" the part named " basalstiick " by Geinitz, 

 and " radicle" by Hall, and he gives a figure of it as it has appeared to him in Graptolites priodon. It 

 is here, as in other monoprionidian species, described as a hollow elongated cone with rounded base, and 

 like the rest of the fossil composed of two membranous laminae. The common canal of the graptolite 

 springs from one side of it at a point not far removed from its thicker end, the pointed end of the foot 

 being, as a rule, thrown back on the dorsal side of the canal. Here, again, the views of the author 

 find no support in any British specimens I have examined. 



Having noticed that certain monoprionidian graptolites occur sometimes curved towards the side 

 of the rod, or " dorsal " side, sometimes towards that of the denticles, or " ventral " side, he concludes 

 that these species must have possessed the faculty of spontaneously bending themselves from side to 

 side. 



He has never witnessed anything which can be compared with the structures referred by the 

 American and British observers to the generative system of the graptolite, but he has seen in the 

 same rocks with the graptolites great quantities of spherical corpuscles of from 0"1 to 0'3 ram. in 

 diameter, and apparently surrounded by a double membrane. He does not offer any opinion as to the 

 import of these bodies, but along with them he finds also, in great abundance, slender conical bodies, 

 resembling in all respects, except that the pointed end was continued into an extremely fine whip-like 

 filament, that part of the graptolite which he had already distinguished as the " foot." He regards 

 these as the earliest stage of the graptolite. They appear to be the bodies referred to in the text as 

 first described by Hall under the designation of graptolite " germs ;" Richter believes that he has seen 

 the stem of the future graptolite originating as a bud from a point near their thicker end. 



In such forms as Rastrites, in which the long distant spine-like processes are usually regarded as 

 the denticles with terminal orifice, he considers these processes as a mere armature of the true 



