xliv Introduction. 



symphysis. It does not however appear to be always developed, as 

 usually stated, from a single centre of ossification in the membrane 

 covering" the distal portion of the cartilage (MeckeFs) of the first 

 branchial arch. In the human subject it has been observed to be 

 developed from as many as four centres of ossification, of which one 

 probably corresponds to the dentary bone of the Crocodile, and the 

 other to the splenial. In the terminal segment of the limbs, the 

 digits never consist of more than three phalanges each, except in 

 the true Cetacea, which order also forms an exception to the rule 

 that the terminal digital phalanges are always in Mammals pro- 

 tected by a nail, a claw, or a hoof. 



In addition to, and together with the hairs so characteristic of 

 this class, and found even in some adult Cetacea upon the lips, we 

 find the integument developing structures as various as the vibrissae 

 on the snout of Carnivora ; the scales on the body and limbs of the 

 Pangolins, and on the tail of certain Rodents, and of Ptilocerctis 

 amongst Insectivora ; the spines on many members of the two 

 last-mentioned orders ; the horns as opposed to the horn-cores 

 of the hollow-horned Ruminants; and the horns of the Rhino- 

 ceros. The scutes of the Armadillo are exclusively dermal pro- 

 ductions. Glands of various kinds are found on very various parts 

 of the body. Some of them are known as sebaceous, sudoriparous, 

 lacteal and lacrymal, according to the character of their secretion ; 

 whilst others, which are usually modifications of the sebaceous 

 type, are, according to the locality in which they are situated, 

 known as anal, inguinal, interungular, and preputial. In all 

 Mammalia, with the exception of the Hare, Lepus timidus, a layer 

 of adipose tissue, the pannicidus ad/-^)Ostis, sometimes of great thick- 

 ness, is interposed between the ctctis vera and the subjacent muscles 

 or bones. The teeth, which are developments of the mucous mem- 

 brane continuous with the external integument, are normally limited 

 to the lower jaw below, and to the pre-maxillary and maxillary 

 bones above. They may be absent altogether or replaced by horny 

 plates, as in the Ornithorhynclms. Only a certain number of the 

 teeth, the so-called ' milk teeth,^ are ever replaced in Mammalia 

 after being shed ; and in many Mammals no such replacement has 

 ever been observed. In the Marsupials, none of which are ever 

 edentulous, there is only a single ' dent de remplacement -' the one, 

 namely, which corresponds to the second human premolar. In 



