224 DESCRIPTION OF [Polygastricii. 



in form the preceding, but it is elongated. Its trapezoid 

 form and reticulation, in place of striae, are seen in 

 plate iv., fiy. 1 83 ; they are represented attached to sea- 

 weed (Callithaminum), to which they mostly attach them- 

 selves. As opaque objects, they exhibit great beauty 

 under the microscope ; the different portions, which appear 

 like reticulations, are in this way shewn in rehef. Size 

 l-60th. 



Genus LXVI. Synedra. The yard or ell-measure Ani- 

 malcules. — The members of this genus, when young, are 

 attached by one of their extremities ; but at a later period 

 are often free. They are longer than they are broad j are 

 destitute of a distinct foot or neck-like process, or which, 

 at most, is rudimentary. Their lorica is simple (smooth 

 or striated), siliceous, and in the form of a wand or rod ; 

 (prism-shaped) openings exist at their ends, but none in 

 the middle ; the cluster of ova is sometimes divided into 

 from two to four leaf-like plates, or into purses or lappets ; 

 digestive cells have not been observed. 



304. Synedra ulna {Vibrio bipunctatus,M.) The com- 

 mon Synedra is striated, truncated laterally ; as age ad- 

 vances the lateral ends become dilated ; upon the broad 

 side of the dilated end are seen three obtuse teeth, and 

 two openings between them. These creatures often occur 

 in vast numbers, appearing as a white incrustation, 

 covering the stones on the banks of rivers in summer. 

 Dr. Lancaster has noticed this in the Annan, Dumfries- 

 shire : wherever a mass of gravel was exposed to the air, 

 the surface of the stones appeared thus covered. With a 

 pocket magnifier, they resembled acicular crystals ; using 

 a deeper power, and examining the wet stones on the edge 



