This somewhat enigmatic form is known on the basis of two skeletal remains, both found on the small island of Marie-Galante, an island in the Guadeloupe archipelago; the first one is a subfossil ulna that had been recovered from the Folle Anse archaeological site and the other one is a very much older phalanx recovered from cave deposits that are of Late Pleistocene age.
The Marie-Galante Macaw most likely wasn’t a distinct species but was identical with the Guadeloupe Macaw (Ara guadeloupensis Clark), for which, however, no skeletal specimen exists and which thus still is considered a hypothetical form. 
 Charles A. Woods; Florence E. Sergile: Biogeography of the West Indies: Patterns and Perspectives, Second Edition. CRC Press; Auflage: Subsequent 2001  Monica Gala; Arnaud Lenoble: Evidence of the former existence of an endemic macaw in Guadeloupe, Lesser Antilles. Journal of Ornithology 156(4): 1061-1066. 2015
This species was described in 1937 based on a single tibiotarsus of an adult-sized immature that was found during archeological excavations on St. Croix Island, US Virgin Islands, Lesser Antilles.
The species was subsequently found in an archaeological site in south-central Puerto Rico too. 
The Saint Croix Macaw may have been native to Puerto Rico and its adjacent islands, or it may have been native to the whole Lesser Antilles. All remains, known so far, originate from archaeological contexts, so are from birds that have to be connected to human activities, they may have been hunted to be cooked and eaten, or they may have been kept as pet birds, having been transported from one island to another etc..
This will probably never be resolved.
I personally think that there may once have been only two endemic macaw species inhabiting the Caribbean region, one restricted to the Greater – and one to the Lesser Antilles; and these most likely were descendants of the Scarlet Macaw (Ara macao (L.)), the only red macaw in Middle America.
 Matthew I. Williams; David W. Steadman: The Historic and Prehistoric Distribution of Parrots (Psittacidae) in the West Indies. pages 175–189 in: Charles A. Woods; Florence E. Sergile (eds.): Biogeography of the West Indies. CRC Press 2001  S. L. Olson; E. J. Maíz López: New evidence of Ara autochthones from an archeological site in Puerto Rico: a valid species of West Indian macaw of unknown geographical origin (Aves: Psittacidae). Caribbean Journal of Science 44: 215–222. 2008  James W. Wiley; Guy M. Kirwan: The extinct macaws of the West Indies, with special reference to Cuban Macaw Ara tricolor. Bulletin of the British Ornithologist’s Club 133(2): 125-156. 2013
The Montserrat Macaw is known from a nearly complete coracoid that had been recovered from the Trants archaeological site on the island of Montserrat and appears to have been indeed a distinct, yet undescribed species.
The species was nearly the same size as the Chestnut-fronted Macaw (Ara severus (L.)), that is about 50 cm. 
 Charles A. Woods; Florence E. Sergile: Biogeography of the West Indies: Patterns and Perspectives, Second Edition. CRC Press; Auflage: Subsequent 2001
The Guadeloupe Macaw aka. Lesser Antillean Macaw is the best presented Caribbean macaw species, regarding contemporaneous accounts.
The first of these accounts dates from 1553, comes from the Spanish historian Gonzalo Fernández de Oviedo y Valdés and is itself referring to another account from 1496, made by Fernando Colón (Ferdinand Columbus), a Spanish bibliographer and the second son of Cristoforo Colombo (Christopher Columbus), who again mentions chicken-sized parrots, which the Island Caribs called Guacamayas, in Guadeloupe. 
There are very detailed accounts made by Jean Baptiste Du Tetre in 1667, who not only describes the bird in detail but also gives some information about its life and the way it was hunted by the natives and so on. 
A subfossil terminal phalanx, found in late Pleistocene cave deposits on the island of Marie-Galante, a small island offshore the east coast of Basse Terre and Grande Terre, Guadeloupe, has been assigned to a Ara sp., another skeletal remain; a single subfossil ulna recovered from an archaeological site on the same small island is also assignable to a Ara sp.. These two remains are the only evidence for the former presense of a macaw species on the Guadeloupe archipelago. 
The St. Croix Macaw (Ara autochthones Wetmore), which is known from several subfossil remains, as well as the undescribed Montserrat Macaw (Ara sp.) known from subfossil remains from the island of Montserrat, might be identical with this species.
References:  J. B. Du Tetre: Histoire Générale des Antilles Habitées par les François. Paris: T. Lolly 1667  Austin H. Clark: The Lesser Antillean Macaws. The Auk 22(3): 266-273. 1905  Charles A. Woods; Florence E. Sergile: Biogeography of the West Indies: Patterns and Perspectives, Second Edition. CRC Press; Auflage: Subsequent 2001  Monica Gala; Arnaud Lenoble: Evidence of the former existence of an endemic macaw in Guadeloupe, Lesser Antilles. Journal of Ornithology 156(4): 1061–1066. 2015
The Caribbean may once have been home for numerous parrot species, including several endemic macaw species, whose former existence, however, is still somewhat questionable; yet the Cuban Macaw is the only Caribbean macaw whose actual existence cannot be doubted.
There are at least 19 museum specimens and some subfossil material to prove the former existence of this small macaw species. 
The Cuban Macaw was only about 50 cm long, it was mainly red, had a yellow neck and blue wings and a blue tipped tail.
The bird inhabited the main island and the Isla de la Juventud offshore Cuba’s southwest coast, and it is in fact quite possible that this species also inhabited to islands of Hispaniola and Jamaica.
The Cuban Macaw was already restricted to the Zapata Peninsula and to the Isla de la Juventud, when it was discovered by European scientists.
The last records date to the 1850s.
The bird was a host of several species of parasites, including the feather mite Distigmesikya extincta Pineda & Ortiz and the feather louse Psittacobrosus bechsteini Mey, which were obviously restricted to this single species and which are now extinct together with their only host. 
 James W. Wiley; Guy M. Kirwan: The extinct macaws of the West Indies, with special reference to Cuban Macaw Ara tricolor. Bulletin of the British Ornithologist’s Club 133(2): 125-156. 2013