The four metacarpals are slightly expanded proximally and distally. Although measurements of length and width are tabulated below (Table 2), we are not certain of the sequence of these bones in the row.
The dimensions of the two proximal phalanges are alike. The shape of these elements is similar to that of the metacarpals. The two terminal phalanges are somewhat triangular in shape, the lateral edges being concave and the proximal convex.
Table 2.—Approximate Measurements of the Forelimb (in mm.)
| Element | Dimensions | |||
|---|---|---|---|---|
| Length | Width | |||
| Proximal | Midway | Distal | ||
| Humerus | 16.0 | 5.0 | 2.0 | 7.5? |
| Radius | 9.0 | 4.0 | 1.5 | 3.5 |
| Ulna | 8.5 | 4.5 | 1.5 | 3.5 |
| Radiale | 3.0 | 2.0 | 1.5 | 2.0 |
| Intermedium | 1.5 | — | 2.0 | — |
| Ulnare | 3.5 | 2.0 | 2.0 | 2.5 |
| Metacarpal A | 4.5 | 2.5 | 1.0 | 2.0 |
| Metacarpal B | 4.5 | 3.0? | 1.5 | 2.5 |
| Metacarpal C | 4.0 | 2.0 | 1.5 | 2.0 |
| Metacarpal D | 3.5 | 2.5 | 1.0 | 1.5 |
| Proximal Phalanx A | 2.0 | 1.5 | 1.0 | 1.5 |
| Proximal Phalanx B | 2.0 | 1.5 | 1.0 | 1.5 |
| Terminal Phalanx A | 1.5 | 1.5 | 1.0 | 1.0 |
| Terminal Phalanx B | 1.5 | 1.5 | 1.0 | 1.0 |
COMPARISONS AND DISCUSSION
Apparently primitive rhipidistian characters in Hesperoherpeton are: Braincase in two sections, posterior one containing an expanded notochordal canal; lateral series of mandibular bones closely resembling that of Megalichthys, as figured by Watson (1926); tabular having long process probably articulating with pectoral girdle; lack of movement between head and trunk correlated with absence of occipital condyle; sensory pits present on frontal and squamosal.
Although we are unable to separate, by sutures, the vomers from the palatines, the palatal surface of these bones and of the pterygoids is studded by numerous small teeth, as in Rhipidistia (Jarvik, 1954) and some of the early Amphibia (Romer, 1947). The stapes apparently reaches the quadrate, and could therefore serve in hyostylic suspension of the upper jaw.
The pectoral limb has an axial series of bones carrying hooklike flanges on their posterior edges. The other bones of the limb show little modification of form beyond the nearly flat, aquatic type seen in Rhipidistia. No distinct elbow or wrist joints are developed.
Characters of Hesperoherpeton common to most primitive Amphibia, in contrast with Crossopterygii, are: Nares separated from edge of jaw; stapes having external process that may have met a tympanic membrane, thus giving the bone a sound-transmitting function. Apparently none of the opercular series was present.
There are two large palatal teeth, slightly labyrinthine in character, adjacent to each internal naris. The scapulocoracoid, as shown by Peabody (1958), is Anthracosaurian in structure, as are the long-stemmed clavicles. The limbs have digits rather than fin-lobes, although the digital number apparently is four and the number of bones in the manus is less than would be expected in a primitive amphibian. The vertebrae are similar to those of Ichthyostegids, as described by Jarvik (1952), except that the pleurocentra are much larger.