OF THE DIATOME^. 81 



the rivers, streams, and lakes of the district I traversed. In these I detected 

 130 sjDecies, and but one form not yet determined as indigenous to Britain. 

 If this be the case with a district much of whose Phanerogamous flora is so 

 different from our own, it bears out the \iew I have taken, that these or- 

 ganisms enjoy a range of distribution far more general than the higher orders 

 of plant -life. 



'' Nor is the distribution of marine species less notable for its extent and 

 uniformity. Coscinodiscus eccentrkus and C. 7'adiatus range from the shores of 

 Eritain to those of South Africa. Gmmmatopliora marina and G. macilcnta are 

 found in almost every marine gatheiing from the Arctic Ocean to the Mauri- 

 tius. Stauroneis pidchella, Cocconeis SciiteUiim, and Bkldulj[>hia pulcliella are 

 equally abundant on the Eiu'opean, the American, and the African coasts, 

 while lihabdonema Adriaticum belies its name by its occurrence in the Indian, 

 Atlantic, and Pacific Oceans. During the researches already mentioned, in the 

 South of France, I made several prolific gatherings on the shores of the Gulf 

 of Lyons; but, of 33 fonns occurring in these, HyaJosira deUcatida, Kiitz., 

 was the only one not famihar to me as a British species." 



The supposition that many species of Diatomeae occupy a very limited geo- 

 graphical area, and that considerable numbers have, in course of ages, disap- 

 peared or become extinct, as many animal and vegetable organisms have done, 

 was thus ably examined by the lamented Dr. Gregory in a communication to 

 the Royal Society of Edinburgh, made in 1856 (Proc. Boy. Soc. Edin. 1856- 

 57, p. 442). The subject of discussion is introduced in his notice of Ma- 

 vicida prcetexta, a form previously considered only fossil. " I have," he says, 

 *' selected this form because the bed in wliich it occurs fossil is the oldest in 

 which Ehrenberg has found any Diatoms. He has indeed found microscopic 

 organisms in the chalk, and even in older rocks, among which he mentions 

 the mountain Hme^tone and the Silurian greensand. But the forms in the 

 two latter rocks are not numerous, and, as well as those Avhich abound in the 

 chalk, belong to the Foraminifera or to the Polycystina, not to the Diato- 

 macea .... In short, I have no hesitation in saying, that I believe all the forms 

 in the ^gina clay-marl, which is the -oldest Diatomaceous deposit yet de- 

 scribed, will be found living on our coast." The stratum at -^gina belongs 

 either to the chalk formation, or to the oldest tertiary or Eocene beds. 



Dr. Gregory continues, " It may also be observed that, of all the forms 

 figured by Ehrenberg from more recent strata, whether mioeene, like the bed 

 on which the town of Richmond (Virginia) is built, and several kinds of Bcrg- 

 mehl — or phocene, like other Berg-mehls or pohshing- slates, &c. — or stiU 

 more recent, the great majority are perfectly identical wdth existing Diatoms. 

 Indeed, although many forms are stated in Ehrenberg's earhest writings to 

 be fossil only, and have been supposed to be extinct, the progress of obser- 

 vation is continually adding to the number of species which are found also in 

 the recent state. Thus, for example, the whole group of dentate Eunotke, 

 which abound in the Lapland and Finland Berg-mehls, were long thought to 

 be only fossil ; but they have been nearly aU found in America, and I have 

 myself seen several of them recent in this countiy. Eunotia triodon, long 

 supposed to be extinct, occurred scattered in many of the Scottish freshwater 

 gatherings. 



" Taking these facts into consideration, I am led to beheve that we have no 

 evidence that any species of Diatom has become extinct, as so many species, 

 and even genera and tribes, of more highlj' organized beings have done. I 

 obser\^e that Mr. Brightwell expresses a similar opinion in his valuable paper 

 on Chcetoceros (J. M. S. iv. p. 105)." 



Wherefore Dr. Gregory comes to the conclusion, that '' the whole of the 



G 



