I think this is  a Tetraedron sp.

It is Staurastrum

I would to confirm: it is a Staurastrum spec.

Can you send a more clear pics?

I would vote for Tetraedon as it looks like more irregular and polygonal than Staurastrum. Furthermore, on this picture, there is no visible isthmus/constriction in between the two semi-cells (=taxonomic feature of Desmids).

It is Staurastrum.

It looks like the xantophyceae: Isthmochloron  ( but i always work with O.M. not with E.M)  (Isthmochloron lobulatum)

In the past  was also called Tetraëdron or Pseudostaurastrum 

Classification:

Empire Eukaryota

Kingdom Chromista

Phylum Ochrophyta

Class Xanthophyceae

Order Mischococcales

Family Pleurochloridaceae

Genus Isthmochloron

It is Staurastrum sp. 

MORE INFORMATION:

Cells of Pseudostaurastrum lobulatum (also known as Isthmochloron lobulatum) which might be confused with some biradiate Staurastrum(e.g., S. bibrachiatum). Scale bar = 10 µm.

http://www.desmids.nl/info/lookalikes/desmid_lookalikes.html

Desmid lookalikes

Although, because of their remarkable morphological symmetry, desmid algae usually can be recognized in itself at once, there are some representatives of other algal groups that resemble them in a deceptive way.

Examples of such confusing non-desmids particularly occur in the algal group of the  xanthophytes. Xanthophytes are characterized by other  photosynthetic pigments than chlorophytes (to which the desmids belong), but under the microscope there is usually no significant difference in colour to be seen. Another essential difference between xanthophycean desmid lookalikes and true desmids  is in the way of reproduction. Xanthophytes do not sexually reproduce by means of conjugation (no zygospores). As, however, sexual reproduction stages in most desmid species are (extremely) rare, this is not a workable discriminating characteristic either. In practice, xanthophycean desmid lookalikes best can be recognized by their chloroplasts. Whereas true desmids in principle have two large chloroplasts per cell, each of them filling in a semicell, cells of xanthophytes contain (many) more than two, relatively small, discoid chloroplasts, distributed all over the cell. As a consequence, the midregion of a xanthophyte cell is coloured by pigments, in contrast to a transparent midregion (isthmus) in true desmid cells.

Two commonly occurring xanthophycean desmid lookalikes are Isthmochloron trispinatum and Pseudostaurastrum lobulatum.

Isthmochloron trispinatum originally was described as a desmid, i.e.Arthrodesmus trispinatus (West & West, 1902).  Skuja (1948) transferred it to the xanthophyte genus Isthmochloron.

Image © Henk Schulp

Cell of Isthmochloron trispinatum. Notice that the isthmial region is filled in with pigment particles.

Cell dimensions ca 15 x 15 µm

Image © Koos Meesters

Another cell of Isthmochloron trispinatum. When it is empty, it can easily be confused with a desmid cell, e.g. of some small-sized Xanthidiumspecies.

The xanthophyte genus Pseudostaurastrum, as the name already indicates, may be confused with the desmid genus Staurastrum.

Images after Peerapornpisal (2005).

Cells of Pseudostaurastrum lobulatum (also known as Isthmochloron lobulatum) which might be confused with some biradiate Staurastrum(e.g., S. bibrachiatum). Scale bar = 10 µm.

Thanks for the information. This is not pseudostaurastrum, but look like since the image is not clear. i will uload some other images soon. I am doubtful whether it is staurodesmus or arthrodesmus?. 

I think this is not a desmid. I cannot see the isthmus in the cell. If the cell is really formed by two semi-cells, you need to upload better quality images which allow to observe this morphological characteristic.

It is a Desmid and also few chrysophyte members lookalike desmids. Staurastrum bibrachiatum i guess so. Kindly refer www.desmid.nl and also, classic monograph, West & West (5 volumes), A Monograph of British Desmidiaceae. Staurodesmus and Arthrodesmus will be lacking in its much diverged arm-like processes. Thank You.

is Isthmochloron