536 PROFESSOR GREGORY ON 



and even oftener. Thus it happens, that I have compared as many specimens of 

 by far the greater number of the forms here mentioned as rare, as if they had 

 been very frequent, and I had only seen one or two slides. 



Compared with many forms, all the Complex Amphorse would be considered 

 rare, but of these, I have in every instance examined numerous specimens, and 

 have satisfied myself of the constancy of their characters, which is the most im- 

 portant point. 



I trust, therefore, that Naturalists will accept this paper as a simple contri- 

 bution to our knowledge of Diatomaceous Forms. As such I present it, leaving 

 to those who are better qualified for the task than I am, to decide on the con- 

 flicting claims of genera and species. 



APPENDIX. 



For the following description and figure of a very beautiful new form, belong- 

 ing to a new genus, I am indebted to my friend Dr Greville. The form in 

 question has not occurred to me as yet, but as Professor Walker- Arnott has 

 found it in the Clyde, it has a claim to be inserted in this account of new Clyde 

 forms. 



Notice of a New Genus of DiatomacecB. By R. K. Greville, LL.D., F.R.S.E., &c. 



My friend Professor Gregory having permitted me to introduce in this place 

 the description of a new and most interesting diatomaceous form, I gladly avail 

 myself of the privilege. Having been recently discovered in the Clyde, it may, 

 indeed, be considered as possessing some claim to appear in company with the 

 multitude of fine species described by Professor Gregory in the preceding pages. 

 This remarkable Diatom was communicated to me a few weeks ago, by my friend 

 Professor Walker- Arnott, for publication and illustration. It is to be regretted 

 that he did not undertake this office himself; he has, however, very kindly sup- 

 plied me with notes of his views regarding it, so that he has rendered my labour 

 comparatively light. Professor Walker-Arnott's attention was first directed to 

 the form in question by the Rev. R. Creswell, who obtained it at Teignmouth, 

 from the stomach of Cynthia rustica {Phallusia rustica, Flem.), along with Bid- 

 dulphia Baileyi and other good things. It is, however, so scarce, that at present 

 it must be reckoned among the rarissima of its tribe. Very fortunately the speci- 

 mens which have been obtained are in a state to admit of satisfactory description. 

 Like Biddulphia and Isthmia, it forms chains, the links or frustules of which are 

 oblong, somewhat depressed at the ends, highly cellulate, separating transversely 



