GENERAL CHARACTERS 231 



relate to the prehension of food generally, are essentially adaptive 

 and consequently plastic or prone to variation, and hence can- 

 not well be relied upon as tests of affinity. In another character, 

 also adaptive, the laxity of the connection of the ribs with the 

 vertebral column and with the sternum, and the reduction of that 

 bone in size, allowing great freedom of expansion of the thoracic 

 cavity for prolonged immersion beneath the water, the Mystacocetes 

 have passed beyond the Odontocetes in specialisation. On the other 

 hand, the greater symmetry of the skull, the more anterior position 

 of the external nostrils and their double external orifice, the form 

 of the nasal bones, the presence of a distinctly developed olfactory 

 organ, the mode of attachment of the periotic bone to the cranium, 

 the presence of a caecum and the regular arrangement of the 

 alimentary canal, the more normal characters of the manus and the 

 better development of the muscles attached to it, and the presence, 

 in many species at least, of parts representing not only the bones 

 but also the ligaments and muscles of a hind limb, 1 all show less 

 deviation from the ordinary mammalian type than is presented by 

 the Odontocetes. Taking all these characters into consideration, it 

 does not appear reasonable to suppose that either type has been 

 derived from the other, at all events in the form in which we see 

 it now, but rather that they are parallel groups, both modified in 

 different fashions from common ancestors. 



Among the Mystacocetes, in the especially distinguishing 

 characters of the division, the Rorquals are less specialised than the 

 Right Whales, which in the greater size of the head, the length and 

 compression of the rostrum, the development of the baleen, and 

 shortness of the cervical region, are exaggerated forms of the type, 

 and yet they retain more fully some primitive characters, as the 

 better development of the hind limb, the pentadactylous manus, 

 and the absence of a dorsal fin. Both types are found distinct in 

 a fossil state at least as far back as the early Pliocene age, but 

 generally represented by smaller species than those now existing. 

 Some of the Pliocene Rorquals (Cetotherium) were, in the elongated 

 flattened form of the nasal bones, the greater distance between the 

 occipital and frontal bone at the top of the head, and the greater 

 length of the cervical vertebra?, more generalised than those now 

 existing. In the shape of the mandible also, Van Beneden, to 

 whose researches we are much indebted for a knowledge of these 

 forms, discerns some approximation to the Odontocetes. 



Among the last-named group there are several distinct types, of 

 which that represented by Platanista, although in some respects 

 singularly modified, has been considered to present on the whole 

 approximations towards the more normal and general type of 



1 These have been described in detail by Professor Struthers in the Journal of 

 Anatomy and Physiology, 1881. 



