Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.1.6

7.5.1.6

sa yaḥ kūrmo 'sau sa ādityo | 'mumevaitadādityamupadadhāti taṃ purastātpratyañcamupadadhātyamuṃ tadādityam purastātpratyañcaṃ dadhāti tasmādasāvādityaḥ purastātpratyaṅ dhīyate dakṣiṇato 'ṣāḍhāyai vṛṣā vai kūrmo yoṣāṣāḍhā dakṣiṇato vai vṛṣā yoṣāmupaśete 'ratnimātre 'ratnimātrāddhi vṛṣā yoṣāmupaśete saiṣā sarvāsāmiṣṭakānām mahiṣī yadaṣāḍhaitasyai dakṣiṇataḥ santsarvāsāmiṣṭakānāṃ dakṣiṇato bhavati

Now this tortoise is the same as yonder sun: it is yonder sun he thus lays down (on the altar). He lays it down in front with the head towards the back (west): he thus places yonder sun in the east looking thitherwards (or moving westward); and hence yonder sun is placed in the east looking thitherwards. On the right (south) of the Ashâdhâ (he places it), for the tortoise (kûrma, masc.) is a male, and the Ashâdhâ a female, and the male lies on the right side of the female;--at a cubit's distance, for at a cubit's distance the male lies by the female. That Ashâdhâ is the consecrated queen (mahishî) of all the bricks, hence being on the right (south) side of her, it (the tortoise) is on the right side of all the bricks.