<p>In 25 B.C. M. Agrippa built a Stoa of Poseidon, τὴν στοὰν τὴν τοῦ Ποσειδῶνος, in the *Campus Martius to honor his naval victories (Dio Cass. 53.27.1); prominently displayed in the portico was a painting of the Argonauts (Dio Cass., <i>loc. cit.</i>). Exactly where this stoa stood and what architectural form it took are heavily debated questions, as the literary and archaeological evidence is quite complex.</p> <p>Post-Augustan authors refer to the Stoa of Poseidon by several different names. A scholiast on Juvenal (6.153) notes that a painting of the Argonauts within the <i>porticus Agrippiana</i> was hidden by temporary stalls for merchants of luxury goods; this certainly refers to the *Saepta and its rôle as a marketplace (but may or may not equal the <i>porticus Agrippae</i> of Hor., <i>Epist</i>. 1.6.26). Martial, in a poetic description of the Saepta, refers to the son of Aeson, that is to say, Jason (2.14.5-6); later he writes of the colonnade of ‘the fickle captain of the first ship’, again a reference to Jason and the Argonauts (11.1.12, transl. D.R. Shackleton Bailey), and even mentions <i>spatia Argonautarum</i> (‘rooms of the Argonauts’: 3.20.11). Gatti was the first to assemble these passages and identify the Stoa of Poseidon with the long W portico of the Saepta, which he termed the <i>porticus Argonautarum</i>, a name which has become familiar in the scholarship (Gatti 1940, esp. 65; Tortorici; Guidobaldi); supplying that name were the 4th-c. Regionary Catalogues which place the <i>porticus Argonautarum et porticus Meleagri</i> (known to be the E portico of the Saepta), together with the <i>basilica Neptuni</i> (<i>Curiosum</i> only), in <i>Regio IX</i>. Thus ancient literary sources establish the Stoa of Poseidon as the W portico of the Saepta, record multiple names for the structure (<i>porticus Argonautarum</i>, <i>porticus Agrippiana</i>; cf. Tortorici 46 for comparable instances of multiple appellations) and distinguish it from the post-Augustan <i>basilica Neptuni</i>.</p> <p>Despite this clear distinction between the Stoa of Poseidon and the <i>basilica Neptuni</i>, several scholars have sought to associate the Agrippan stoa with Hadrian’s restoration of the <i>basilica Neptuni</i> (SHA, <i>Had</i>. 19.10; Cordischi, Coarelli, Castagnoli), a rectangular vaulted hall S of the *Pantheum (remains are visible from Via della Palombella: Ghini, Cordischi). However caution needs to be exercised when associating this basilica with the building of Agrippa as the extant portions date in their entirety to the 2nd c. A.D. and, further, represent a distinctly different architectural form — a basilica — rather than the attested Agrippan stoa.</p> <p>Two lines of argument have been advanced to support identifying the Hadrianic remains as a reflection of an earlier Agrippan structure. First, Hadrian restored (<i>instauravit</i>) the <i>basilica Neptuni</i> (SHA, <i>loc. cit.</i>); thus some assume that the basilica had an Agrippan predecessor (Castagnoli, Ziolkowski), and overlook the fact that <i>instauravit</i> only indicates a pre-Hadrianic predecessor — not necessarily an Agrippan one (perhaps Hadrian restored a Flavian Basilica of Neptune, built during Domitian’s restoration of the Pantheon, as proposed by Cordischi 15). Second, Ziolkowski argued that the Hadrianic <i>basilica Neptuni</i> retained the footprint of an Augustan-era structure since its remains seemed identical in size and orientation to the Agrippan Pantheon. But recent excavations uncovered a new portion of the Agrippan Pantheon, and its <i>pronaos</i> is now known to have been considerably deeper than Ziolkowski’s figures (by <i>c</i>. 7 m); this and the confirmation of its N entrance severely undercut Ziolkowski’s thesis (s.v. Pantheon).</p> <p>More ambiguous is the possible affiliation of the Stoa of Poseidon with the <i>Poseidonion</i> (τὸ Ποσειδώνιον), a structure which burned in the same fire of A.D. 80 that took the Saepta, Pantheon, *Diribitorium, and Thermae of Agrippa (Dio Cass. 66.24.2). Both names have been linked with the Thermae Agrippae and Pantheon in monument lists, thus the two should be associated with the same region of the Campus Martius. This and the similarity of their names suggest that they designated the same edifice. Yet, at 66.24.2, Dio treats the Saepta and the <i>Poseidonion</i> as two distinct structures, which initially suggests that the <i>Poseidonion</i> and Stoa of Poseidon were separate entities. However, Dio’s distinction actually offers little assistance since in antiquity the porticoes which flanked the Saepta were regarded both as independent buildings and as subsidiary portions of the Saepta (Richardson 340). A possible solution is advanced by Cordischi (1990, 11), who posits two distinct structures: the Stoa was a portico built by Agrippa while the <i>Poseidonion</i> was a temple, possibly of Agrippan patronage (contra, Gatti 1940, 67-68, who holds that the two designate the same structure and that <i>Poseidonion</i> has no religious connotations). While Tortorici, like Cordischi, finds that the term ‘<i>Poseidonion</i>’ denotes a sacral structure, he reaches an opposite conclusion (based upon analysis of the naming conventions for architecture in Dio) when he considers the Stoa of Poseidon and Poseidonion to be alternate names for a portion of the W portico of the Saepta characterized by a religious dedication, perhaps an altar or cult statue (46-47). The fact that the Senate was able to convene in the Saepta in 17 B.C. during the <i>ludi Saeculares</i> (<i>CIL</i> VI 32323.5) confirms that the building was a <i>templum</i>, and thus may lend some credence to Tortorici’s hypothesis, but until new evidence is presented the <i>Poseidonion</i> can only tentatively be identified with the Stoa of Poseidon.</p>