170 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



seen in section from below is smaller than that of a large Lemur varius. It consists of two coils, a much 

 larger medial coil, extending from the fenestra cochleae inward, downward and forward, and then upward, 

 outward and backward, and a smaller lateral coil, surrounding the apical portion of the osseous core or 

 modiolus. A natural cast of these two coils is preserved in Amer. Mus. No. 12569 (A'", osborni). It shows 

 that there was little if any part of a third turn at the apex, indeed it is very doubtful whether even two 

 turns of the spiral were entirely completed, while in Lemur a very small septum near the apex indicates 

 part of a third turn. Between the coils in Notharctus three septa were represented as we pass from behind 

 forward: a thick medial septum separating the outer coil of the cochlea from the hypotympanic sinus, 

 a middle septum, springing from the region of the carotid canal and passing inward, and an outer septum, 

 coiled around the spiral tip of the modiolus. The lamina spiralis ossea is represented by a low ridge seen 

 on the posteromedial concavity of the middle septum; it is less developed than in Propithecus (Plate 

 XLVI). 



In general, the cochlea of Notharctus, like that of Lemur, if considered as a coil resting on its base, 

 seems to have a smaller base in proportion to the height of its axis ; while in Propithecus and still more so 

 in Ateles, representing the Platyrrhini, the base is much wider in proportion to its height. This gives 

 the latter the appearance of a flatter, more closely wound coil, with a lower pitch. In Ateles also the 

 lamina spiralis ossea is sharply defined, the apical portion of the coil is wider and better differentiated, 

 so that the whole coil makes considerably more than two complete turns. The surroundings of the 

 cochlea in Ateles are totally unUke those of Notharctus, Lemur and Propithecus, as the cochlea is covered 

 below not by the hypotympanic sinus but by the cavum tympani. (Plate XLVI.) 



The internal auditory meatus is shown in N. osborni, Amer. Mus. No. 12569, and N. matthewi, Amer. 

 Mus. No. 13030. Its only noteworthy feature is that it is larger than that of Lemur varius and more 

 extended transversely ; the subarcuate fossa above it and behind it on the contrary is not as deeply ex- 

 cavated as in the modern Lemur and Propithecus; a slender canal in the posterodorsal wall of the sub- 

 arcuate fossa seems to be the superior semicircular canal, as in Lemur and Propithecus, but the aperture 

 of the fossa being smaller does not extend up to the canal as in those forms. The most noteworthy feature 

 in this region is the primitive exclusion of the mastoid from the cranial cavity, owing to the lack of brain 

 expansion in this region. Above and in front of the internal auditory meatus the cranial surface of the 

 periotic forms an eminence, separating the anterior from the posterior parts of the brain and indicating 

 the beginning of the crista petrosa of Lemur and Propithecus. 



OSSICULA 

 Plate XLVII 



The malleus, incus, and stapes were found in place in the attic, or epitympanic recess, of the right 

 side. The head of the left malleus was also found in situ. The above named recess comprised a small 

 posterior sinus, containing the malleus and incus, and a much larger anterosuperior sinus which extended 

 dorsad between the squamosal and the periotic, exactly as in Lemur. Stereoscopic views of the ossicles, 

 compared with those of Ateles and Macacus, are shown in Plate XLVII. A study of these specimens 

 and of Doran's memoir on the "Morphology of the Mammalian Ossicula auditus" (1876) shows that 

 they are of primate type, with no special affinities to those of insectivores. 



The malleus of Notharctus on the whole more nearly resembles that of Propithecus coquerelli, with 

 which it agrees in the massive and deeply notched articular surface, which contrasts with the wide but 

 shallow articular surface in Ateles. It approaches Ateles in the marked subcircular expansion of the 



