Recently the French anatomist Robineau (1968) drew attention to the existence of a third occipital condyle in a skull of the Beaked Whale Mesoplodon bidens (Sowerby, 1804). That condyle, the remainder of the pro-atlas hypocentrum, was among the Mammalia known only in a few primates. In the skull of M. bidens the very small third occipital condyle was functional, viz. it articulated with the ventral part of the atlas. The French scientist also mentioned a skull of Mesoplodon densirostris (de Blainville, 1817) in which a still smaller and non-functional third condyle was present.

On February 13, 1971, a full-grown female Strap-tooth Whale, Mesoplodon layardii (Gray, 1865), (total length 554 cm.), was stranded on the beach at East Clive near Napier, New Zealand. When the skull of this animal was cleaned, a small but functional third occipital condyle was observed (see fig. 1). The following day, another Beaked Whale was stranded on the Haumoana Beach near Napier. This was found to be an adult female specimen of Mesoplodon grayi von Haast, 1876 (total length 498 cm.); the skull of this specimen also showed a third occipital condyle (see fig. 1).

Although the number of observations is still much too small for any definite inference, nevertheless the general impression is that the anomalous condylus tertius is not extremely rare in specimens of the genus Mesoplodon, as it has now been found in the skulls of four different species.

Figure 1: Third condyles ventrally between the occipital condyles of Mesoplodon layardii (at right) and of Mesoplodon grayi (at left). Distance between outer limits of the condyles in M. layardi 126 mm.