BULLETIN FLORIDA STATE MUSEUM metapodials. As in more advanced horses, these specimens of N. phlegon do not have sagittal ridges on the distal articular surfaces. (Stirton [1940] noted that faint sagittal ridges are developed on primitive species of "Nannippus. ") The great similarity in the distal trochlea of the medial (III) metacarpal and metatarsal makes it difficult or nearly impossible to distinguish between the first medial phalanx of the manus and pes. The classic work on equid digital ligaments by Camp and Smith (1942) and later studies (Sondaar 1968; Hussain 1975) showed these phalanges have many characters of functional significance. UF 2427 and UF 22632 are first phalanges of the medial (III) metapodial (an ad- ditional specimen, UF 17548, is discussed in Robertson 1976). These phalanges are relatively long and slender (Fig. 13, Table 10). Sondaar (1968) noted that in the manus the length of the first phalanx increases from about 1/6 that of the medial (III) metacarpal in Mesohippus to about 1/3 that of the medial (III) metacarpal in Equus. In the two UF specimens of N. phlegon the length of the first phalanx is about 1/4 the length of the medial (III) metapodials. The proximal articular surface is concave with a deep groove to ac- commodate the well-developed sagittal ridge on the distal trochlea of the medial (III) metacarpal. This deep groove and well-defined ridge system is characteristic of advanced horses and is related to restric- tion of lateral movement in the fetlock. In.more primitive horses lateral movement was somewhat restricted by the presence of more elongate lateral metapodials (Sondaar 1968). In this regard N. phlegon is more like one-toed horses, for example Equus, than it is to other three-toed horses, such as Hipparion. A B b bD 0 1 2 3 4 5cm FIGURE 13.-Medial (III) phalanges of Florida Nannippus phlegon from Santa Fe 1A. First phalanx, (A) dorsal view; (B) volar or plantar view; a, V-scar; b, oval perforatus scars. Second phalanx, (C) dorsal view; (D) volar view. Vol. 25, No. 1