445 



9. Navicula Barclayana, n. sp. 



16. Navicula Sirithii, var y, nitescens. 



17. „ Sinithii,var.S^suborbicularis. 



18. „ maxima Greg. 



19. Pinnularia subtilsi, n. sp. 



20. „ rostellata, n. sp. 



21. „ Allmaniana, n. sp. 



22. „ Pandura, Breb, var. 



I have given a figure of the typical N, Lyra^ Ehr., not yet 

 figured as a British form. The figure referred to by Professor Smith 

 in his second vol. (vol. i. fig. 152^) is that of the variety /?. 



I exhibit drawings of all the most remarkable of these forms, and 

 an enlarged figure of N. 'prcetexta, Ehr., which is not only remarkable 

 and beautiful in itself, but interesting from the circumstances in 

 which it occurs. 



Ehrenberg found it only fossil, in the Clay Marl of ^gina, where 

 it seems to be very scarce, as he has figured an imperfect specimen. 

 I have found it recently, both in Lamlash Bay and Loch Fine, and 

 though not abundant, yet sufficiently frequent to have enabled me 

 to distribute a good many specimens. No doubt, if we persevere in 

 examining the estuarial deposits, we shall some day find it in greater 

 abundance in the vicinity of its proper habitat. 



Here, then, is a form which, till now, has been regarded as fossil 

 only, which is found to be still existing in the Clyde. The Clay 

 Marl of ^gina is stated by Ehrenberg to belong either to the 

 chalk formation, or to the oldest tertiary or eocene beds. I have 

 selected this form, because the bed in which 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 Limestone and the Silurian 

 Green Sand. But the forms in the two latter rocks are not nume- 

 rous, and, as well as those which abound in the chalk, belong to the 

 Foraminifera or to the Polycystinese, not to the Diatomaceas. 



We find, then, that this very remarkable form, which occurs fossil 

 in the Clay Marl of .^gina, exists in the Clyde at the present day ; 

 and there is no difference whatever between the fossil and the recent 

 specimens. 



But this is not all. In the same Clay Marl Ehrenberg has 

 figured many more species of Diatoms, and of these, upwards of three- 

 fourths are absolutely identical with forms which abound in the 

 Clyde. Such are Navicula Bombus, N, incurvata, Pinnularia 

 Pandura, Orthosira marina, Amphitetras antediluviana, Tricer- 



VOL. III. 2 o 



