On the characteristic Fossils of the Chalk Formation. 381 



row subterminal black-brown band ; tail and underside of body 

 white, scarcely black varied ; whiskers thick, black, white at the 

 base ; upper cutting-teeth with a distinct subcentral longitudinal 

 groove. 



Young. Fur ? 



Cercolabes prehensilis, var. Waterhouse, N. H. Mamm. ii. 414. 



The skull of the Bolivian specimen is much larger, wider over 

 the orbits and much higher from the palate to the nose and 

 forehead than in the Brazilian specimens : the grinders are con- 

 siderably smaller, and it has the peculiar groove on the upper 

 grinders, but the latter may be an accidental or individual pecu- 

 liarity. 



The following measurements in inches and lines of three 

 skulls in the Museum collection will show these peculiarities. 

 No. 1 is the skull of the Bolivian specimen ; No. 2 that of the 

 Brazilian specimen : these animals are nearly of the same size. 

 No. 3 is a skull of a skeleton from the Brazils. 



No. 1. No. 2. No. 3. 



in. lin. in. lln. in. lin. 



Skull : Length, entire 4 2 3 5 '6 7\ 



Width at orbit 2 4 2 2 2 3 



over orbit 2 4 1 6'i 1 8i 



at nose 1 H 11 Hi- 

 Height from palate to tip of nose ... 1 5 1 li 12 



from palate to top of forehead . 2 2 19^ 1 10 



of teeth series 9^ 9^ 10 



There is a specimen in the Museum which Mr. Waterhouse 

 has described as a variety (Hist. Mamm. ii. 415). It is very di- 

 stinct in appearance from either of the above, but best agrees with 

 the specimen from the Brazils in the blackness and slenderness of 

 the whiskers and the smoothness of the upper cutting-teeth, and 

 the blackness of the tip of the tail, but differs in the general co- 

 lours being much blacker, and in the underside of the body and 

 tail being nearly black and only very slightly grizzled, and espe- 

 cially in the tips of some of the spines on the sides being yellow. 

 I strongly suspect it will prove a third species, to which the 

 name of C. tricolor might be attached. 



XXXV. — On the characteristic Fossils of the Chalk Formation. 

 By L. Von Buch*. Communicated by Prof. J. Nicol. 



Throughout all the members of the chalk formation, three chief 

 forms of organic beings seem especially adapted to serve as cha- 

 racteristic fossils. These are the Ammonida, the Trigonice, and 



* From Betrachtungen iiber die Verbreitung und die Grenzen der Kreide- 

 Bildungen. Bonn, 1849. 



