1888.] 



On the First and Second Visceral Arches. 



143 



The shoulder-girdle is described and restored, and found to consist 

 of interclavicle, clavicles, sternum, coracoids, pre-coracoids, and 

 scapulae. The scapula is in plan like Kistecephalus. The nearest 

 approach to the coracoid and pre-coracoid is found among the mono- 

 treme mammals. The clavicle extended along the anterior margin 

 of the scapula, and made an angular bend, so as probably to meet the 

 interclavicle. The interclavicle appears to meet the lateral margins 

 of the coracoids and not to overlap them in front. It approximates 

 in form to the bone in Ornithorhynchus, Ichthyosaurus, and certain 

 lizards, but is relatively much larger, and is larger than the inter- 

 clavicle of Stereorachis. The sternum, which is transversely extended, 

 is better compared with that of a lizard or Dinosaur. The shoulder- 

 girdle as a whole is intermediate between monotreme mammals and 

 known reptiles, but with the former type predominating. 



The bones of the fore-limb are described in detail, and found to be 

 relatively long and slender and generically unlike Dicynodon. The 

 carpus is complicated. There are only two phalangeal bones in each 

 digit, the second bone being a well-developed claw. 



Finally a restoration is given of the aspect of the animal. 



VI L M On the Modifications of the First and Second Visceral 

 Arches, with especial Reference to the Homologies of the 

 Auditory Ossicles." By Hans Gadow, Ph.D., M.A., Strick- 

 land Curator and Lecturer on Comparative Anatomy in 

 the University of Cambridge. Communicated by Professor 

 M. Foster, Sec. R.S. Received April 12, 1888/ 



(Abstract.) 



The phylogenetic development of the first visceral arches shows us 

 some most interesting changes of function, which we can follow 

 upwards from the lower Selachians to the highest Mammals. 



Originally entirely devoted to respiration as gill-bearing structures, 

 the whole hyoidean arch becomes soon a factor in the alimentary sys- 

 tem. Its proximal half forms the hinge of the masticatory apparatus, 

 its distal half remains henceforth connected with the process of deglu- 

 tition. Then this suspensorial arrangement is superseded by a new 

 modification ; the hyomandibula is set free and would disappear (it 

 does nearly do so in Dipnoi and certain Urodela), unless it ffere 

 made use of for a new function ; w T ith its having entered the service of 

 the conduction of sound, it has entered upon a new departure, and it 

 is saved from degeneration. The whole system of the one to four 

 elements of the middle ear, which all have the same function as con- 

 ductors of sound, is to be looked upon as one organ of one common 



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