1286 
the species Cebus and Ateles living in the same country. FroweRr *) 
determined the brain weight of an exceedingly emaciated old male 
Mycetes seniculus, which had died in the London Zoological Gardens, 
at 48 grams (740 grains), the body weight at 3444.5 grams (9 lbs. 
9*/, ozs. avoirdupois). Spirzka’) found the brain weight of a 
female Mycetes ursinus (which species is somewhat larger) to be 
54 grams, Lreuw®) found 63 cm’. for the cranial capacity of Mycetes 
ursinus in an adult male, and 54 em°. in an adult female specimen. 
The body weight was only known of atrophical zoological garden 
individuals. From Surinam [ received the skull and other parts of 
the skeleton of a male Mycetes seniculus, killed in the natural 
state, which weighed 6750 grams, though judging from the condition 
of the skeleton, it was only almost full-grown. The cranial capa- 
city is 54 em*., from which a brain weight of 50 grams can be 
calculated and a cephalisation-coefficient 0.37, about the same as 
that of Macacus cynomolgus. For the entirely full-grown state a 
still somewhat lower value would certainly have been found *). 
The Howling Monkeys, now, are described as being, in their free 
state, exceedingly indolent animals, which remain very much at the 
place where they are. All their movements are slow, almost creeping ; 
they never play with each other, climb deliberately, and never 
jump far—in sharp contrast to the lively, rapid movements, the 
leaps and swings of the agile rovers of the genera of Cebus and 
Ateles. The cephalisation coefficient of these is more than three 
times as great as that of Mycetes. 
Here, therefore, the same contrast as between swift and slow 
species of Reptiles and Amphibians. Thus Hyla arborea has double 
the cephalisation of Rana fusca. And as it is demonstrated there 
(e.g. between Phrynosoma and Sceloporus) it may be assumed here 
that the nerve fibers (and the muscle fibers) are thicker, the neurones 
more voluminous in the more vigorous and quicker species ®). 
1) W. H. FLower, On the Brain of the Red Howling Monkey (Mycetes seniculus 
Linn.) Proceed. Zool. Soc. London. 1864, p. 335—338. 
2) E. A. Sprrzka, Brain-Weights of Animals with Special Reference to the Weight 
of the Brain in the Macaque Monkey. Journal of Comparative Neurology. Vol. 13, 
p. 13, Philadelphia 1905. 
3) W. Lecue, Ueber Beziehungen zwischen Gehirn und Schädel bei den Affen. 
Zoologische Jahrbiicher. (SPENGEL). Supplement XV, Band 2, p. 17. Jena 1912. 
4) What is urgently required is more data of body weights in the 
free state. Especially nimble animals get much lighter in captivity; the 
brain weights change less, and can also be calculated pretty accurately from 
the cranial capacity. 
5) Euc. Dusois, “The Significance of the Size of the Neurone and its Parts.“ 
These Proceedings, Vol. XXI, No. 5, p. 711. 
