Sri Kurma natha which is the second avatar of Lord Vishnu, Kurma avatara (Tortoise) and this is the only temple of Kurmanatha in India.

The moolasthanam (central piece) is considered to be large saligrama. The Kurma(Turtle) is known as Sri Kurmam in the ‘Sampradayam’. The image is not a sculpture crafted by man, but the fossil of an actual (large-size) turtle, which must have paddled on to the ‘kshetram’ where the shrine has been built. The head of the diety is in the form of a Kurma (tortoise) and is represented by a Vishnu namam, the tail being represented by a Saligrama presented by Adi Sankaracharya. The artistic beauty of the temple lies in the carvings of Vishnu and other diety statues in a glorious way of architecture. There is a mandapam in front of the temple on Lion pillars. One unique feature of this temple is that it has Dhwajastambams in the front and back of the temple. That is because the deity faces the back of the temple on the west.

Legend : According to inscriptions in the temple dated 1281 A.D, the holy place of Kürmakshetra, was actually reestablished by Sri Ramanujacharya under the influence of Jagannatha Deva at Jagannatha Puri, Orissa. Later the temple came under the jurisdiction of the King of Vijayanagara. The deity was worshiped by the Vaishnavas of the Madhvacharya Sampradäya.
Read more : http://balsanskar.com/english/lekh/265.html