146 BuUctin American Museum of Natural History. [\o\. XXVII, 



Spencer and Sweet (1899) on certain Marsupials show that some hairs are 

 from the first not radiaUy symmetrical, as usually in mammals, but flattened 

 dorso-ventrally, with the cortical layer thicker on the dorsal surface. 



(2) Pari passu with the reduction of the scales, the heavy spines may 

 have been replaced by finer and finer spines, just as the strong-spined Echidna 

 aculeata typica grades into the hairy Echidna acukata setosa (Thomas, 

 1885). 



(3) If spiny hairs are a primitive mammalian character this may pos- 

 sibly explain their frequent retention and progressive specialization in the 

 very lowly orders Insectivora and Rodentia. 



The former coexistence and intermingling of hairs and scales in ancestral 

 mammals is also supported by numerous facts: 



(1) The coexistence of hairs and true "reptilian" scales in Mcuiis 

 (Weber, 1894). The primitive nature of these scales has been questioned 

 by Beddard (1902, p. 189), but defended by Weber (1904, pp. 420-421) \ 



(2) The mingling of hairs and minute scales in the foetal Dasypus and 

 in the tail of DidelpJiis, Petrogale and many Insectivores and Rodents. The 

 tail, being of little adaptive importance in the economy of many animals 

 might be expected to retain in some instances very ancient hereditary char- 

 acters (p. 334). 



(3) The mingling of hairs and a scale-like pattern of the epidermis in 

 the manus and pes of many Insectivores and Rodents. 



(4) The fact that when hairs and scales are intermingled the hairs are 

 usually grouped together in threes. Hence in other cases de Meijere (1894) 

 regards the occurrence of "Dreihaargruppen" alone as a vestige of the 

 former coexistence of hairs and scales (Weber, 1904, pp. 11, 12). 



Reptilian Characters in the Reproductive Organs. . The Origin 



OF Lactation. 



The completely Sauropsidan character of the female genitalia and the 

 retention of oviparity in the Monotremes are too well known to require 

 special comment, but there is one character in the male organ that may be 

 cited here which shows how far below the remaining mammals the Mono- 

 tremes stand: namely the fact that the penis has not yet acquired its double 

 function of ejecting both the urinary and reproductive products, but trans- 

 mits the latter alone (Weber, 1904, p. 326). In the Monotremata also the 

 ureters o]:)en into the urinogenital canal, while in the Theria they open into 



1 In view of the completely mammalian nature of Manis in respect to other characters, 

 and of its numerous highly aberrant specializations, it would seem more likely that the scales 

 are entirely secondary. 



