1867.] 259 [Wyman. 



peated in different animals ; in the first case he calls them Iwmotypes 

 and in the second homologues. 



As a general rule, homologous parts resemble each other in form 

 and uses, and by these are easily recognized. But there are instances 

 in ■which form and use are insufficient for the determination of the 

 homology of a given part. The "prickly pear" (^Opuntki) Avhich has 

 the appearance of being all leaves, is in reality all stem, and this has 

 not only the form but the function of leaves. Two homologous bones 

 from different animals may be so unlike that an anatomist might be 

 excused for taking them for different parts ; and on the other hand, 

 two different parts may be so nearly alike that were one not on his 

 guard they would be considered homologous. Even Cuvier, in his 

 earlier days, mistook the coracoid of a turtle for the scapula, on ac- 

 count of its shajje.* Among teeth, molars and premolars interchange 

 forms. Even here Cuvier was misled by this circumstance and de- 

 scribed the large back teeth of the feline carnivora as true molars. 

 Owen has shown that these supposed molars were the successors of de- 

 ciduous teeth and therefore premolars. Human anatomists have gen- 

 erally misunderstood the homology of the articulating surfaces of the 

 atlas and the upper part of the axis ; and from their use have de- 

 scribed them as if they were " articulating processes," while in man 

 and most mammals, these processes do not exist in the vertebrfe men- 

 tioned. True, articulating processes are found in the Cetacea, and 

 some birds and reptiles, co-existing with the articulating surfaces 

 above mentioned, and occupying the true position of articulating 

 processes which the "surflxces" do not. We might extend the list of 

 such instances, but it is unnecessary. 



As the form and use of a given pai't under certain circumstances 

 may leave us in doubt as to its homology, we need some other 

 guide. This may be found, as GeofFroy St. Hilaire long since pointed 

 out, in the relative position. The tusks of the elephant and mastodon 

 are only known to be incisors by their position in the intermaxillary 

 bones, and the radius and ulna of Icthyosaurus and Plesiosaurus are 

 identified by their relation to the humerus and carpus, and not by 

 their forms or uses. Had these been the only parts of the above 

 mentioned animals which had been discovered, naturalists would have 

 hardly sus^aected them to be ulna and radius. So in the determination 

 of homotypes we are not to expect precise similarity of forms, as these 

 are liable to differences analogous to those of homologues, as is 

 especially the case in the carpus and tarsus. 



If we are justified in accepting the conclusions set forth in this pa- 

 per, then by an application of them the homotypes in the two limbs 



* Legans sur I'Anat. Comp. l" edit. T. i, p. 252. 



