SCIENCE. 



65 



bottom of the containing vessel. There it will remain 

 motionless until it dies ; but if it be again transferred to sea 

 water it will recover, provided that its exposure to the fresh 

 water has not been of too long duration. I have never 

 known a naked-eyed Medusa survive an exposure of fifteen 

 minutes : but they may survive an exposure of ten, and 

 generally survive an exposure of five. But although they 

 thus continue to live for an indefinite time, their vigor is 

 conspicuously and permanently impaired. While in the 

 fresh water irritability persists for a short time after spon- 

 taneity has ceased, and the manubrium and tentacles are 

 strongly retracted. 



Turning now to the case of the freshwater species, when 

 first it is dropped into sea water at 85 there is no change in 

 its movements for about fifteen seconds, although the ten- 

 tacles may be retracted. But then, or a few seconds later, 

 there generally occurs a series of two or three tonic spasms 

 separated from one another by an interval of a few seconds. 

 During the next half minute the ordinary contractions 

 become progressively weaker, until they fade away into 

 mere twitching convulsions, which affect different parts of 

 the bell irregularly. After about a minute from the time of 

 the first immersion all movement ceases, the bell remaining 

 passive in partial systole. There is now no vestige of 

 irritability. If transferred to fresh water after five minutes 

 exposure, there immediately supervenes a strong and per- 

 sistent tonic spasm, resembling rigor mortis, and the animal 

 remains motionless for about twenty minutes. Slight 

 twitching contractions then begin to display themselves, 

 which, however, do not affect the whole bell, but occur 

 partially. The tonic spasm continues progressively to 

 increase in severity, and gives the outline of the margin a 

 very irregular form ; the twitching contractions become 

 weaker and less frequent, till at last they altogether die away. 

 Irritability, however, still continues for a time — a nip with 

 the forceps being followed by a bout of rhythmical contrac- 

 tions. Death occurs in several hours in strong and irregu- 

 lar systole. 



If the exposure to sea wa'er has only lasted two minutes, 

 a similar series of phenomena are presented, except that the 

 spontaneous twitching movements supervene in much less 

 time than twenty minutes. But an exposure of even one 

 minute may determine a fatal result a few hours after the 

 Medusa has been restored to fresh water. 



Contact with sea water causes an opalescence and essen- 

 tial disintegration of the tissues, which precisely resemble 

 the effects of fresh water upon the marine Medusae. When 

 immersed in sea water this Medusa floats upon the surface, 

 owing to its smaller specific gravity. 



In diluted sea water (50 per cent) the preliminary tonic 

 spasms do not occur, but all the other phases are the same, 

 though extended through a longer period. In sea water 

 still more diluted (1 in 4 or 6) there is a gradual loss of 

 spontaneity, till all movement ceases, shortly after which 

 irritability also disappears ; manubrium and tentacles ex- 

 panded. After an hour's continued exposure intense rigor 

 mortis slowly and progressively develops itself, so that at 

 last the bell has shrivelled almost to nothing. An exposure 

 of a few minutes to this strength places the animal past 

 recovery when restored to fresh water. In still weaker 

 mixtures (1 in 8, or 1 in ten) spontaneity persists for a 

 long time; but the animal gradually becomes less and less 

 energetic, till at last it will only move in a bout of feeble 

 pulsations when irritated. In still weaker solutions (1 in 

 12, or 1 in 15) spontaneity continues for hours, and in solu- 

 tions of from 1 in 15 to 1 in 18 the Medusa will swim about 

 for days. 



It will be seen from this account that the freshwater Me- 

 dusa is even more intolerant of sea water than are the ma- 

 rine species of freshwater. Moreover the freshwater Medusa 

 is beyond all comparison more intolerant of sea water than 

 are the marine species of brine. For I have previously 

 found that the marine species will survive many hours' im- 

 mersion in a saturated solution of salt. While in such a 

 solution they are motionless, with manubrium and tentacles 

 relaxed, so resembling the freshwater Medusa shortly after 

 being immersed in a mixture of 1 part sea water to 5 of 

 fresh ; but there is the great difference that while this small 

 amount of salt is very quickly fatal to the fresh-water 



species, the large addition of salt exerts no permanently 

 deleterious influence on the marine species. 



We have thus altogether a curious set of cross relations. 

 It would appear that a much less profound physiological 

 change would be required to transmute a sea-water jelly- 

 fish into a jelly-fish adapted to inhabit brine, than would 

 be required to enable it to inhabit fresh water. Yet the 

 latter is the direction in which the modification has taken 

 place, and taken place so completely that sea water is now 

 more poisonous to the modified species than is fresh water 

 to the unmodified. There can be no doubt that the modifi- 

 cation was gradual — probably brought about by the ances- 

 tors of the freshwater Medusa penetrating higher and higher 

 through the brackish waters of estuaries into the fresh water 

 of rivers — and it would I think be hard to point to a more 

 remarkable case of profound physiological modification in 

 adaptation to changed conditions of life. If an animal so 

 exceedingly intolerant of fresh water as is a marine jelly- 

 fish may yet have all its tissues changed so as to adapt them 

 to thrive in fresh water, and even die after an exposure of 

 one minute to their ancestral element, assuredly we can see 

 no reason why any animal in earth or sea or anywhere else 

 may not in time become fitted to change its element. 



George J. Romanes. 



A NEW GENUS OF RHINOCERONTID^E. 



While the genus Aphelops must be regarded as the direct 

 ancestor of the recent rhinoceroses with canine and incisor 

 teeth, now confined to Asia and the Islands, the ancestral 

 genus of the African forms and their extinct congeners, 

 which are without the teeth named, is less known. It can 

 now be shown that the missing genus inhabited North 

 America, and that, like Aphelops, it is hornless. It may be 

 named and characterized as follows : Peraceras, Cope ; supe- 

 rior dentition ; I. o ; Co; P-m. 4 ; M. 3 ; nasal bones 

 weak, hornless. 



This genus is established on a new basis recently dis- 

 covered by Mr. R. H. Hazard, in the Loup Fork formation 

 of Nebraska, which may be called Peraceras superciliosus. 

 It is founded on a nearly perfect skull, which lacks the 

 lower jaw. Its size is about that of the Indian rhinoceros. 

 It is narrowed anteriorly, but is very wide between the 

 orbits. Posterior to these it contracts rapidly, and rises to 

 a rather elevated occiput. Saggital crest narrow ; a promi- 

 nent angle above each orbit. The premaxillary bone is 

 narrow and weak. The nasal notch extends to above the 

 middle of the third superior premolar. The occiput is rec- 

 tangular in outline, with truncate summit. Its surface 

 above is concave, divided by a strong median crest ; lower 

 down a vertical groove intersects its lateral border. The 

 crests of the molar teeth are rather simple, and the poste- 

 rior notch is soon isolated on attrition. Wear also isolates 

 an external median fossa of the second premolar. Length 

 of skuil from end of premaxillary bone to condyles, M. 700 ; 

 length of alveolar border of premaxillary, .025 ; length of 

 molar series, .315 ; length of three true molars, .160; width 

 of crown of second true molar at base, .075 ; superciliary 

 width, .255. 



This species is nearest to the Peraceras malacorhinus, a 

 species which I foimerly referred to Aphelops, but which I 

 have little doubt belongs to the present genus. It differs 

 from P. superciliosus as follows : In the latter species the 

 front is wider, and is plane or concave, not convex ; the 

 superior edge of the maxillary is not wide and incurved, 

 and has not the oblique ridges ; the infraorbital foramen 

 consequently has a more lateral opening. The nareal notch 

 does not extend so far p steriorly by the one and a half 

 molar teeth. The occiput is wider, is divided by a median 

 crest not found in P. malacorhinus, and has the vertical 

 lateral grooves much shorter. Tne acute supraorbital angle 

 is not seen in the P. malacorhinus. 



The rhinoceroses of the Loup Fork formation whose 

 generic position can now be ascertained, are the following: 

 Paraceras malacorhinus ; P. superciliosus ; Aphelops meridi- 

 anus ; A. Negalodus ; A. fossiger. — Am. Naturalist. 



