40 



THE QUARTERLY REVIEW OF BIOLOGY 



vertebrae. A similar history is known 

 for living cetaceans. Prozeuglodon stromeri 

 is the representative of this series in the 

 Upper Eocene. A fairly complete skele- 

 ton (Stromer, 1908), measuring about 

 ten feet in length including the skull, 

 which measures 2.8 inches, gives us a 

 fairly adequate conception of this species. 

 Several diverse types of zeuglodont 

 skulls, which appear to belong to 

 four distinct series, have been found in 

 the Upper Eocene. "Zeuglodon" osiris 

 (Stromer, 1903) is probably the best 

 known of the small zeuglodonts that make 

 their appearance subsequent to the lower 

 Middle Eocene. The skull of this species 

 did not exceed 30 inches in length. The 

 braincase is narrow, elongated, and high 

 posteriorly, and the bones in the cranium 

 retain their normal mammalian relation- 

 ships. The skull of "Zeuglodon" osiris 

 is remarkable for the narrowness of its 

 occipital shield, the lateral portions of the 

 lambdoid crest being sharply deflected 

 backward so that this surface has the 

 appearance of being constricted above the 

 condyles. These modifications afford an 

 increase in surface for attachment of jaw 

 muscles and no doubt are correlated with 

 increasing size and strength of the man- 

 dible. As compared with "Zeuglodon" 

 intermedius the condyles and foramen 

 magnum are smaller, the rostrum seems to 

 be narrower, and perhaps the opening 

 leading to the nostrils was situated farther 

 back. In this series there is a tendency 

 toward a narrowing of the occipital shield 

 accompanied by a folding back of the 

 lambdoid crests, resulting in increasing 

 concavity of the surface above the foramen 

 magnum. The sagittal crest increases in 

 height, and the condyles and foramen 

 magnum diminish in size. The cheek 

 teeth of these two zeuglodonts are serrated 

 on their anterior and posterior cutting 

 edges, and the last upper molar is lost. 



According to Andrews (1913) the buttress 

 or postero-internal cusp is reduced in size 

 and apparently was not supported by a 

 third root. 



The depression of the occipital shield in 

 the Zygorhiza minor series was accompanied 

 by decreasing convexity of the lateral 

 cranial walls and a rather pronounced 

 folding backward of the lambdoid crest. 

 This crest is exceptionally high and flaring, 

 and is raised from 30 to 40 mm. above the 

 lateral walls of the braincase. The ros- 

 trum has increased in length and slender- 

 ness, and the opening leading to the 

 nostrils has moved farther backward, the 

 interval between it and the orbit being 

 about one-third the length of the rostrum. 

 In Zygorhiza minor (Miiller, 1849, and True, 

 1908), however, both ends of the 

 premolar-molar series appear to have been 

 reduced, inasmuch as the first upper single- 

 rooted premolar and the third upper molar 

 are missing on the cast of the type skull 

 obtained from the Tyler museum. The 

 first tooth of the upper premolar series of 

 "Zeuglodon" %itteli according to Stromer 

 (1903, p. 82.) is likewise two-rooted. In 

 the smaller zeuglodonts, such as Zygorhiza 

 minor and ProZeuglodon stromeri (Stromer, 

 1908, No. Mn. 9, p. no, pi. 4), the skull 

 equalled approximately one-fifth of the 

 total length of the skeleton, but in the 

 colossal types like Basilosaurus cetoides not 

 more than one-tenth of the total length. 



The carnivorous zeuglodonts typified 

 by Basilosaurus cetoides (Gidley, 1913) of 

 the Gulf Coastal Plain of the United 

 States, which attained a length of from 50 

 to 70 feet and whose skull measured ap- 

 proximately 5 feet in length, reached the 

 flood tide of their evolutionary advance 

 during late Eocene times. It seems to be 

 generally conceded that gigantism is one 

 of the indications of approaching extinc- 

 tion, but there must have been other 

 contributing factors, for the Oligocene 



