266 



Dr. Gadow. On the 



[Mar. 25, 



intervened between them, is very remarkable, and could only have been 

 attained, however perfect the instrument, by the method of observa- 

 tion which has been described, unless indeed the readings had been 

 taken to fractions of a scale division. 



In the course of my experience in working with the instrument, I 

 have naturally become acquainted with a number of little devices, 

 difficult to describe, which would give me an advantage over a novice 

 in its use. But I believe that any competent manipulator would find 

 it quite easy to obtain uniform and consistent results with it. 



The efficiency of the apparatus is due partly to the perfection of the 

 optical arrangements and partly to the fact that in the moving parts 

 unnecessary lightness has not been acquired at the expense of sufficient 

 massiveness and rigidity. 



III. " Remarks on the Cloaca and on the Copulatory Organs of 

 the Amniota." By Dr. Gadow. Communicated by Pro- 

 fessor M. Foster, Sec. R.S. Received March 11, 1886. 



(Abstract.) 



The sphincter muscles of the anus of Crocodilia are differentiations 

 of the postpelvic portion of the system of the m. rectus abdominis 

 rather than of the true caudal muscles. 



The copulatory muscles of the Carinatae are derived from the m. 

 sphincter ani solely, whilst in the Ratitae they are also differentiations 

 of muscles which are still attached to the pelvis, and are, therefore, 

 skeleto-genital. 



The mammalian sphincter ani does not take a share in the muscle 

 supply of the copulatory organ, and thus exhibits a difference from 

 Birds and Lizards. 



Distinctly copulatory muscles in the Mammalia are derived from 

 skeletal and from non-striped muscles. In this respect the Mammalia 

 agree with Crocodilia and Chelonia. 



Then the author describes the nerve-supply of the cloacal region in 

 Crocodilia. 



Third Chapter. — The modifications of the cloaca in the various chief 

 groups of Amniota : Crocodilia, Lizards, Snakes, Hatteria, Birds, 

 Tortoises, Mammals. 



Lizards and snakes together represent a special type. 



Hatteria comes nearest the Amphibia, or the embryonic condition of 

 Sauropsida ; bears, hoAvever, resemblance to the Lizards. 



Chelonia represent a type somewhat intermediate between that of 

 the Ratitae and Crocodilia and that of the Monotremata, at the same 

 time bearing slight resemblance to that of the Sauria. 



