HISTORY OF WHALES 



187 



carina; elongate zygomatic processes, 

 which do not reach forward to the level 

 of the apex of the supraoccipital shield; 

 and the vertex of the braincase as viewed 

 from behind is strongly depressed below 

 the arching lambdoid crests. This imper- 

 fect cetothere skull is quite important, 

 for it demonstrates that telescoping was 

 an accomplished fact by the close of the 

 Oligocene and that an advanced stage in 

 the forward thrust of posterior occipital 

 elements, such as is found in Cetotheriopsis, 

 must have been preceded by developmental 

 stages that led up to this type of telescop- 

 ing. The Oligocene period unquestion- 

 ably witnessed great elaboration of the 

 telescoping process in the cetotheres, but 

 unfortunately their known fossil remains 

 are so scant that they give us no clew as to 

 how this extraordinary condition came 

 about. The scarcity of fossil cetaceans in 

 Oligocene formations seems all the more 

 surprising in view of the wide variety of 

 types that make their appearance in 

 formations of Lower Miocene age. 



Two distinct types of cetothere skulls 

 from the Lower Miocene Patagonian 

 marine formation of Chubut Territory, 

 Argentine Republic, have been obtained. 

 One of these, Cetotheriopsis moreni (Lydek- 

 ker, 1894), consists of a skull measuring 

 74 inches in length, a mandible, tympanic 

 bullae, and vertebrae. Though originally 

 assigned to the genus Cetotherium it is 

 obvious that this allocation is incorrect, 

 as is evidenced by the absence of anterior 

 temporal crests, the extreme flatness of 

 the cranium, the triangular supraoccipital 

 shield extending forward beyond the 

 level of the posterior margins of the 

 supraorbital processes, the very broad 

 maxillaries, and the elongated nasal 

 bones situated almost entirely anterior 

 to the preorbital angles of the supra- 

 orbital processes. This cetothere is 

 tentatively referred to the genus Ceto- 



therio-psis on account of obvious similarities 

 in the development and extent of the 

 supraoccipital shield. Cetothere skulls 

 with supraoccipital shields of similar 

 proportions also occur in more recent 

 geological stages. 



The other Patagonian skull "Plesio- 

 cetus" dyticus (Cabrera, 19x6) exhibits a 

 second type of occipital shield. The 

 inclination of the supraorbital processes 

 appears rather unusual at first sight, but 

 this abnormality is largely the result of 

 distortion from rock pressure. The supra- 

 occipital shield is distinctly constricted 

 near the apex. As compared with 

 Cetotberiopsis moreni the braincase is nar- 

 rower at the base, the shield is more 

 rounded, and the lambdoid crests are 

 more salient, but the vertex is depressed 

 as in the former. The forward movement 

 of the supraoccipital pushed the parietals 

 in front of it, so that they overspread the 

 posterior borders of the frontals in the 

 interorbital region. The sutures on the 

 anterior borders of the frontals indicate 

 that the nasal bones and the median 

 rostral elements were slightly interdigi- 

 tated with the cranium. The extent of 

 the forward overthrust of the supra- 

 occipital shield and the backward inter- 

 digitation of the mesial portion of the 

 rostrum, the unusually slender supra- 

 orbital processes, the position of the nasal 

 bones, as well as the relations of the bones 

 in the intertemporal region show that this 

 cetothere represents another advanced 

 stage in the telescoping of the mysticete 

 skull. 



In one or more phyla of mysticetes the 

 telescoping of the skull seems to have 

 been largely limited to a forward move- 

 ment of the posterior cranial elements, for 

 the interdigitation of the rostral and 

 cranial elements is very slight. This 

 condition prevails in skulls of the Helve- 

 tian "Mesocetus" hungaricus (Kadic, 1907) 



