T. hispida var. duplex Deflandre 1926? There are experimental work done on the degree of mineralization in Trachelomonas, may be 30+ years ago. This can depend upon environmental factors..... like chelation and availability of iron and manganese (redox)
I think Dag and Sandip has given a fairly accurate answer to your question. However, looking at the picture, I am skeptical as to whether the organism Trachelomonas in the first place.
The pictures in the pdf presented by Mr. Chakraborty shows examples of T. hispida with spines equally distributed over the lorica and your picture doesn't. I am quite sure that it is Trachelomonas, but perhaps it could also be an example of T. armata with underdeveloped caudate spines?