BRITISH DIATOMACEjE. 63 



7- Orthosira ? mirabilis, n. sp. Filaments direct ; valves trans- 

 versely marked by curved costae (?), which occasionally anastomose 

 at the apices of the curves ; line of junction distinctly spinous ; 

 spines very short, subdistant. Breadth of filament *00ll". v.s. 

 Fresh water. Near Haverfordwest, South Wales, April 1855, Mr. Okeden. 



This remarkable form has occurred only to the discoverer, and in small 

 quantities. It unfortunately reached me too recently to be figured in the 

 present work, and in a state which does not enable me to describe it as fully 

 or satisfactorily as its singular character deserves. 



Genus 51. MASTOGLOIA, Thwaites. 



Frustules oblong, annulate, definite, aggregated or solitary ; the 

 mucus, secreted during self-division, more or less persistent as a 

 mamillate cushion or frond ; valves naviculoid ; annuli loculated ; 

 loculi opening by foramina along the line of suture. 



This genus introduces us to the second tribe of the Diatomaceae, 

 in which the frustules are invested with a gelatinous or membranous 

 envelope, and, by their aggregation, form a more or less conspicuous 

 frond. The frustules of Mastogloia are notably distinct from those 

 of any of the other genera of the tribe, having the annulate structure 

 described under the genus Rhabdonema, with the conspicuous cana- 

 liculi of a Surirella. In the present case, the canaliculi, which take 

 the form of loculi, are, however, formed differently from those of 

 Surirella, not being connected with the valve, but with the annulus, 

 which projects as a septum into the body of the frustule. This 

 structure will be best understood from a careful examination of the 

 details, given in Plate LIV. fig. 341. The S. V. of the desiccated 

 frustule, as seen with direct light, is given at a ; the same under 

 oblique light, and focused for the surface only, at a': fig. 341 x 100 

 gives the valves in situ ; and fig. 34 If shows the annulus with its loculi 

 at A & h. The size and capacity of the loculi are often conspicuous 

 in balsam mountings, the difficulty of expelling the contained air 

 and filling the chambers with balsam giving the appearance of 

 empty spaces, as seen at 340 a'. Normally, the annular septum 

 extends only partially across the interior of the frustule, but occa- 

 sionally the loculi are seen to reach nearly as far as the median 



