Disk circular (dd= 4.46 to 5.80 mm). Covered by numerous small and imbricating scales (Fig. 4a). Radial shields slightly longer than wide, with external margin convex and internal margin straight, united except at proximal ends, where one or two small triangular and elongate scales separate them (Fig. 4a). Ventral interradius covered by scales slightly smaller than dorsal scales (Fig. 4b). Bursal slits narrow. Oral shields arrow-shaped (Fig. 4c). Madreporite with one or two pores at distal margin. Adoral shields narrow and enlarged laterally. Two oral papillae at each side of jaw angle, distal one longer and broader (Fig. 4c). Infradental papillae small. Dorsal arm plate broader than long, rectangular (Fig. 4d). Ventral arm plate pentagonal, wider than long, with a slight notch in distal margin. Three arm spines enlarged, compressed and blunt, the ventral one being the longest (Fig. 4e). Two small, perpendicular, tentacle scales.

Florida, the islands off southern Florida, the Antilles, Panama Brazil, and off Mar del Plata, Argentina (Tommasi 1970, Bernasconi and D’Agostino 1977, Hendler et al. 1995, Alvarado et al. 2008). In Brazil from Maranhão, Ceará, Rio Grande do Norte, Paraíba (Albuquerque 1986), Bahia (Thomas 1962, Magalhães et al. 2005), Espírito Santo (Albuquerque and Guile 1991), Rio de Janeiro (Von Martens 1867, type locality) and São Paulo (Tommasi 1970). Depth 0-300 m. In the present study collected between 11 and 27m.

This species is known from bottoms of sand, mud, gravel and algae (Tommasi 1970). It is found burrowed in the sediment together with other ophiuroids such as Ophiophragmus pulcher H.L. Clark, 1918 and Amphioplus albidus (Ljungman, 1867) in Florida and Hemipholis elongata (Say, 1825) in Brazil (Hendler et al. 1995). Thomas (1962) showed that the shape of the arm spines (compressed and blunt) and the noncontiguous adoral shields are important characters to separate Amphiodia planispina from the other more closely related species such as Microphiopholis atra (Stimpson, 1854). Thomas (1962) remarked that these morphological characters may not be present in all specimens, as was also observed by us. The specimens observed in this study differed from the description provided by Tommasi (1970) only in relation to the number of scales between the radial shields. The specimens (dd = 8.5 mm) analized by Tommasi showed two to seven scales between the radial shields, while the specimens of this study (dd = 5.80 mm) had one or two scales. This fact is probably related to size of the specimens of both studies.