87 



the very short beak, for the slender, cone-like teeth, and in the living 

 state for the round, almost globular top of the head, indicating its 

 distinct nature and an approach towards the Globiocephalidse. 



Oecaella fluminaxis,' Anderson, M.S. The Irawady Dolphin. 



Bjnoujms—Bolplin of the Irawady, Anderson, P.Z.S., 1870, pp. 220, 

 544. 



Orcaella fliiminalis, Gray, SuppL, p. 80. 



Body slender, dirty white. 



Inhabits river Irawady, deep channels, from 300 to 1,000 miles from 

 the sea. 



Oenus Oeca,^ Eondelet. The Killer. 



Head rounded, scarcely beaked ; dorsal fin high, falcate, central ; 

 pectoral fin broad, ovate ; skull rounded ; beak short, about one-half the 

 entire length of skuU ; lower jaw strong, thick and solid in front, broad 

 on the sides ; symphysis moderate in length ; teeth large, acute, fiattened 

 transversely, incurved at their tips. 



On examining various skulls of the genera Orca and Pseudorca in 

 our Museum, I was induced, from the variations presented, to analyze 

 the tabulated admeasurements of others of various growths, recorded 

 by Owen, Grray, Flower, Gervais, &c., as well as the excellent illustra- 

 tions in Van Beneden's work on the Cetacea, now in the course of 

 publication. Prom this research I arrived at the conviction that age 

 and sex, assisted by occasional individual peculiarities, have produced 

 many of those material deviations ia the cranial structure which are 

 so pointedly adduced as denoting distinct specific characters. 



In illustration of this assertion, — select from either of these two 

 genera (of course of the same species) the lower jaws of old and young 

 animals, and it wiU. be found by their comparison that ia old age the 

 length of the symphysis, the solidity of the adjoining parts, and the 

 posterior span at the condyles, have respectively assumed proportions 

 greatly in excess of those which might reasonably have been anticipated 

 by a computation derived by rule of three from the condition of the 

 similar parts of the young animal. 



This additional massiveness of bone, and extra width of the grasping 

 power of the mandible, beyond a proportionate increase in its length, 

 would necessitate a corresponding change in the form of the cranium, 

 suflicient to present a marked contrast between the skulls of the very 

 aged and of the young adult. 



^ Jtmninalis, of or belonging to a river. 



' Orca, the name given by Pliny to a large dolpliiiL 



