4IO 



NATURE 



\March 23, 1876 



by the tide or waves, there is no evidence of their volun- 

 tarily leaving the water to bask or feed on the shore. 



The species now existing are very few, and there is 

 reason to believe that the time is not far distant when 

 they will all become extinct. One species, the Rytina 

 stelleri, or Northern Sea-cow, an animal attaining nearly 

 the length of thirty feet, by far the largest known member 

 of the order, from the North Pacific, was totally exter- 

 minated by the agency of man during the last century, 

 and the surviving species, the Manatis and Dugongs, 

 being valuable for their flesh as food, their hides, and 

 especially for the oil obtained from the thick layer of fat 

 which lies immediately beneath their skin, rapidly 

 diminish in numbers as civiHsed populations occupy the 

 regions which form their natural habitat. 



The Manatis (genus Maiiatus), found on the Atlantic 

 coasts of America and Africa, are rather fluviatile than 

 marine in their habitat, ascending large rivers almost to 

 their sources, and feeding chiefly on aquatic grasses. The 

 Dugongs (genus Halicore) are more distinctly marine, 

 feeding chiefly on algae. They inhabit the shallow waters, 

 bays and creeks of various coasts of the Indian Ocean, 

 the Red Sea, East Coast of Africa, the Indo-Malayan 

 Archipelago, and north coast of Australia. There is 

 probably not more than one species, but they have been 

 divided into three according to the locality which they 

 inhabit. H. tabernactili from the Red Sea, H. dugons; 

 from the Indian Seas, and H. australis from Australia. 

 These two existing genera present such well-marked dis- 

 tinguishing characters that if they alone were known they 

 might be placed in separate famihes, but as in so many 

 similar cases our knowledge of the extinct forms, imper- 

 fect as it is, goes far to bridge over the distinction between 

 them. It is true that Brandt, a great authority on this 

 group, divides the order into two primary sections— il/fl/za- 

 tida, consisting of Manatus alone, and Haltcoridce, con- 

 taining all the other genera ; but it scarcely seems that 

 these can be considered in any sense as equivalent, espe- 

 cially as one of the distinguishing chracters, the external 

 form of the tail is unknown in the extinct genera. 



The Miocene and early Pliocene seas of Europe 

 abounded in Sirenians, to which the generic name Hali- 

 therium, Kaup, has been given. They had large tusk-like 

 incisors in the upper jaw, as in the existing Halico7-e, though 



not so greatly developed. Their molar teeth are — or — , 



anteriorly simple and single-rooted, posteriorly with three 

 roots in the upper jaw, and two below, and with enamelled, 

 tuberculated, or ridged crowns, in which respect they ap- 

 proach nearer to Manatus, the molar teeth of the Dugongs 

 being without enamel and single-rooted. The anterior 

 molars were deciduous. Some species, at least had nasal 

 bones, short, broad, but normal in position, whereas in all 

 the existing genera these bones are quite rudimentary. 

 Another and still more important evidence of conformity 

 to the general mammalian type is the better development of 

 the pelvic bone, and the presence of a small styliform 

 femur articulated to the acetabulum, although no traces of 

 any other part of the limb have been discovered. These 

 ancient Sirenians were thus, in dental, cranial, and other 

 osteological characters, less specialised than are either of 

 the existing species, and if the intermediate links could be 

 discovered, might well be looked upon as ancestral forms 

 from which the latter have been derived, but at present 

 the transitional conditions have not been detected. As 

 far as we know, when changes in the physical conditions 

 in the European seas rendered them unfitted to be the 

 habitation of Sirenians, the Halitherium type still pre- 

 vailed. If the existing Dugongs and Manatis are de- 

 scended from them, their evolution must have taken place 

 during the Pliocene and Pleistocene epochs, the one in 

 seas to the east, the other to the west of the African con- 

 tinent, which has formed a barrier to their intercommuni- 

 cation. Halitherium remains have been found in many 



parts of Germany, especially near Darmstadt, in France, 

 Italy, Belgium, Malta, the Isthmus of Suez, &c. Until 

 lately none were known in our own country, probably 

 owing to the absence of the beds of an age corresponding 

 to those in which they are found on the Continent ; but 

 quite recently a skull and several teeth have been detected 

 among the rolled debris of Miocene formations, out of 

 which the Red Crag of Suffolk is partially composed. The 

 species are not yet satisfactorily characterised. Some of 

 them appear to have attained a larger size than the exist- 

 ing Manati or Dugong. One of these from the Pliocene 



of Italy and France, having but — molar teeth, has been 



separated generically under the name of Felwtoiherium by 

 Capellini,by whom it has been fully described. Aportionof a 

 skull found in Belgium has been named Crassithe7-ium, by 

 Van Beneden, and some compressed teeth, somewhat simi- 

 lar to, but larger than those of the Dugong, discovered 

 in the department of Lot et Garonne, France, have given 

 origin to the''genus Rytiodus of E. Lartet. Pachyacanthus 

 of Brandt, from the Vienna basin, is also, according to 

 Van Beneden, another form of Sirenian, of which, how- 

 ever, the skull is not known. In various Miocene and 

 perhaps Eocene marine formations of the United States 

 of America, remains of Sirenians have been discovered, 

 but mostly in such a fragmentary condition that they 

 afford at present little evidence of the early history of the 

 group in that country. A more satisfactory discovery is 

 that of a nearly complete skull and some bones from a 

 limestone tertiary formation in Jamaica. It is of smaller 

 size than the Manati, and as far as the teeth are con- ^ 

 cerned, of a still more generalised character than Hali- I 

 therium, the dentition being apparently " 



i'hcLp lin'k.^\%. 

 3153 

 The incisors are small, not developed into tusks, the ,.. 

 canines (wanting in all existing Sirenians) are rather 

 longer than the incisors, judging by the sockets, and the 

 molars are bilophodent, and covered with enamel. It 

 has been described by Prof. Owen under the name of 

 Prorastomus sirenoides. Unfortunately we have no 

 knowledge of the geological antiquity of the formation 

 in which it was embedded. Lastly must be mentioned 

 the Eotherinm egyptiacum, Owen, founded on the cast of 

 a brain, with a small quantity of surrounding bone, dis- 

 covered in the Nummulitic limestone of Eocene age of 

 the Mokattam Hills, near Cairo. The brain is narrower 

 than that of Manatus, and resembles Halitherium. This 

 is of interest, as the most ancient known evidence of any 

 Sirenian, whose age has been geologically determined. 



The few facts we have as yet been able to collect of the 

 former history of the Sirenians leave us as much in the 

 dark as to the origin and affinities of this peculiar group 

 of animals as we were when we only knew the living 

 members. They lend no countenance to their association 

 with Cetacea, and, on the other hand, their supposed 

 affinity with the Ungulata, so much favoured by modern 

 zoologists, receives no very material support. The as- 

 sumption lately put forth with so much confidence that j 

 the Sirenia are the remains of a group of animals, through I 

 which the Cetacea passed in their evolution from terres- 

 trial Mammalia, is quite without foundation, 



( To be continued. ) i 



PROF. HUXLEY'S LECTURES ON THE EVI- \ 

 DENCE AS TO THE ORIGIN OF EXISTING j 

 VERTEBRA TE ANIMALS ^ 



II. I 



IT was seen in the last lecture that no ultimate answer . 

 was obtainable as to the origin of the examples I 

 selected from the fish class, any more than is afforded as j 



I A course of six lectures to working men, delivered in the theatre of the 

 Royal School of Mines. Lecture II., March G. Continued from p. 309- 



