212 PROCEEDINGS OF THE 



ought to be given of tlie two valves combined, another of the 

 upper valve only after it has been separated by the action of 

 boiling nitric acid from the lower, and a third of the lower valve. 

 Smith says that the markings on the lower valve are less intense, 

 but this is not all the difference : the situation is frequently, if not 

 always, so different, that one might easily suppose the two valves 

 to belong to very distinct species. A familiar instance of this 

 may be seen in C. cUrupta of Gregory. Gregory has only figured 

 and described the upper valve; he says, "the median line is 

 irregular, like a slit or tear down the middle of the external 

 surface:" this is not the median line, but the part of the 

 valve unoccupied by strise, and through which the median 

 line of the lower valve would have been seen had the two 

 been combined. Gregory says that " the C. diaphana occurs along 

 with it, but concludes that the two are not the same thing, since 

 C. cUrupta is by no means diaphanous, while its strise are consi^icuous 

 and its colour brown, the strise of G. diaphana being very obscure, 

 and the valve colourless." Now, what Gregory calls C. diaphana, 

 is not the true C. diaphana of Smith, but is tlie lower valve of C. 

 dirupta. The true C. diaphana has not yet been found in Scotland, 

 nor even on the shore of England, properly so called. Smith's 

 only specimen was from Pontac in Jersey, and j^reserved on mica; 

 it is now in the British Museum; what he describes and figures as 

 the var. /3, from Sidmouth, is, however, the lower valve of C. 

 dirupta. My first acquaintance with C. dirupta was from a 

 gathering made at Plymouth by MrBoswarva in 1853. On send- 

 ing part of this to Mr Smith, he at first, looldng only at the upper 

 valve, pronounced it a new species; but, afterwards, on observing 

 the other one, exactly of the same size and shape, declared all to 

 be C. diaphana. Soon after that Mr E. Kennedy, of Glasgow, 

 found it in a growing state, on Sphacelaria cirrhosa, on the shore of 

 Cumbrae, and a slide from that sample, supplied by me, is now 

 the representative of C. diaphana, among Smith's slides in the 

 British Museum. The true C. diaphana is perfectly different, both 

 valves being diaphanous, and the upper one more convex than in C. 

 pediculus. I may here mention that the true C. diaphana was 

 parasitic on the antheridia of a species of Pohjsiphonia (aj^parently 

 P. nigrescens) ; it had been previously found by M. de Brebisson on 

 the coast of Normandy, at GranviUe, on the antheridia of Gloiosi- 

 phonia Griffithsiana, and called by him Frustulia niduUms; but 



