226 Report of the State Geologist. . 



2. If the colonies were sessile, one would expect to find in the 

 great number of colonies observed, some at least attached to 

 shells, pebbles, etc., for the colonies would probably have pre- 

 ferred fixation to foreign bodies to a mooring in the soft ooze, 

 as do the recent Sertularians.^ 



3. The wide horizontal distribution of the Graptolites and their 

 limited vertical range has made them the basis of a very detailed 

 and persistent division into zones of the Cambrian, Ordovician 

 and Silurian strata such as only the widespread Ammonites have 

 furnished in other ages. Barroisf accounts for this, as well as 

 for their distribution in shale, sandstone and limestone, by their 

 having been floating organisms at an early stage. The writer 

 has observed a few specimens which seem to indicate a floating 

 habit in the siculae. One of these, reproduced in PL III, ^g. 2 , 

 shows two siculiB which give the impression of having been ar- 

 rested by the hydrocaulus, the surrounding surface of the slab 

 being free from siculae. But if the siculae floated, the colonies 

 most probably floated also, as there has not been found any 

 change in the development of Diplograpttis which would indi- 

 cate a change in the mode of life of these organisms. 



On account of these observations, the writer held the opinion 

 which he expressed in the preliminary note, that Diplograpttis 

 was a floating colony. A short time ago, however, a discovery 

 was made which shows evidence not compatible with a floating 

 mode of life. The specimen is a large slab exhibiting at one end 

 upward of a hundred colonial stocks of D. Euedemanni. The 

 latter are all in a fair state of preservation, spread out regularly, 

 about equally distant from each other and arranged in a well- 

 defined area, outside of which only a few broken rhabdosomes 

 are found (cf. PI. Y, which is a representation of part of the 

 slab). The improbability of such an array of nicely ordered, 

 apparently undisturbed stellate groups having been drifted to- 

 gether, is obvious. 



It is further worth mentioning that most of those colonial 

 stocks which show only the central disc and funicle, are sunken 

 in the center, a feature which, it seems, could be easily explained 



* Allman {op. cit., Vol. I, p. 27) says: " In almost every case the general colony, as hydrosoma, is at- 

 tached to some foreign body, such as rocks, shells of mollusca and Crustacea, seaweeds, floating 

 timber, etc., to which it is fixed bysome part of its surface." 



+ Memoire sur la Distrib. des Graptolites en France. 



