MAMMALIA. 403 



In our narrative of the history of each group we shall start from below 

 and ascend the series which we have just presented, commencing accord- 

 ingly with Edentata. In a natural method the extinct groups should 

 always precede the living ones ; but as they are generally less known, they 

 will be found sometimes to follow the latter, and more specially so when 

 doubts are entertained with regard to the family and genera to which they 

 may belong. 



Order I. Edentata. 



The order of Edentata is composed of comparatively few animals, differ- 

 ing widely from each other, but agreeing in the common characters of 

 absence of front teeth or incisors, and the presence of feet that are ungui- 

 culated, that is to say, terminated by large claws or nails. The Edentata 

 existed in larger proportion during the tertiary epoch than in our days, and 

 from considerations derived from palaeontological evidences to be discussed 

 hereafter, we suppose that they have lived during the deposition of the 

 secondary beds, although no remains have yet been found in those deposits. 

 It is also a fact of great importance that during the tertiary period the 

 animals of this order attained to a bulk far surpassing that of any living 

 representatives. 



The Edentata may be divided into three groups, according to certain 

 modifications in their organization, the Monotremata, the Edentata proper, 

 and the Tardigrada. 



Group 1. Monotremata. 



The group of Monotremata has received its name from the peculiarity 

 of having only one external opening for the seminal fluid, urine, and excre- 

 ment, as in birds. They possess the marsupial bones, but have no external 

 pouch in which the young pass one part of their embryonic life, as in Mar- 

 supialia. The mammae themselves have been long a matter of doubt, as 

 well as their mode of generation. According to some travellers these 

 animals lay eggs. But if the mammae do not exhibit a projecting nipple, 

 still the mammary glands have been shown to exist on the abdomen in the 

 form of numerous elongated, sub-cylindrical lobes, converging and opening 

 into a small oval areola. The ear has no external concha. The struc- 

 ture of the skeleton presents many peculiarities by which this family is 

 distinguished from any other. The sternum and shoulder bones join and 

 encircle the fore part of the trunk. Their brain wants the corpus 

 callosum, and the mass called corpora quadrigemina is imperfectly 

 divided. 



This group contains but two families, OrnithorhynchidcB and Echidnidcs, 

 both of them belonging to Australia. 



Fam. 1. ORNiTHORHYNCHiDiE, includcs but onc genus, Ornithorhynchus, 



607 



