NO. 1696. AIR-BREATHING VERTEBRATES— MOODIE. ^3 



j^ersistent judging from the fractured portion of the most anterior 

 centrum preserved. There are evidences of 21 presacral vertebra?, 

 and there may have been 7 or 8 more, thus bringing the number of 

 presacrals near 30, agreeing exactly in this instance with the vertebral 

 column of Mesosaurus from Brazil and also agreeing with Sauravus 

 from France, so far as can be determined. There are two sacral 

 vertebrae in Isodectes. Twenty-two caudal vertebrae are preserved. 

 They are very stout near the base of the tail but taper gradually 

 until, at the tip, the size is reduced to near one-half. There are two 

 lengths in the caudal vertebra?, as may be seen by referring to Plates 

 4 and 5. The sixth vertebra from the tip is nearly one-third as long 

 again as the succeeding one. There is another long vertebra four 

 vertebra^ above this one. Just what the meaning of this condition is 

 I can not determine, since it has not been observed, so far as I know, 

 in any other early reptile. In Mesosaurus the vertebrae are appar- 

 ently of uniform length. The neural canal is plainly visible in some 

 of the distal vertebrae of the tail and this sinks quite deeply into some 

 of the vertebrae, so that one is led to conclude that there are repre- 

 sented the two unconnected halves of the vertebrae ; or, in other words, 

 we have here a primitive condition of the persistence of the pleuro- 

 centra such as is paralleled in the developing vertebrae of the chick 

 of thirteen days' incubation « and also in the vertebrae of some lizards. 



There are evidences of 27 pairs of ribs. They are all intercentral 

 in position, agreeing in this respect with the condition in all the 

 Microsauria so far studied from the Carboniferous of North America, 

 and also with the condition in Sauravus. In Mesosaurus the ribs 

 have migrated backward from the intercentral position. Twenty- 

 four pairs of presacral ribs are preserved. They are all strongly 

 recurved exactly as in Sauravus^ and are stoutly built. They are not 

 distally expanded and are progressively shorter backward. The ribs 

 are apparently single-headed as in Sauraims, there being no evidence 

 of the bicipital condition, although this may be indicated by the 

 expansion of the heads. The three pairs of caudal ribs preserved are 

 short and more strongW recurved than the presacral ribs. The pres- 

 ence of sacral ribs can not be determined. 



The left hand is of such a size that we may safely conjecture that 

 the fore limbs were of some strength, but further than this nothing 

 can be said. The carpus as preserved consists of three distal car- 

 palia, being the carpal elements for digits II, III, and IV. There are 

 two small ossicles lying beside the phalanx of digit III, which may 

 be other carpalia. The first digit is preserved entire and the meta- 

 carpals of the other digits are also preserved. The phalangeal ele- 

 ments are all alike in the absence of any great amount of endochon- 

 drium. There are three elements in the first digit, the metacarpal 



"Lillie, Development of the Chick, 1908, p. 426, fig. 242. 



