466 OSBURN 



there has never been any real evidence adduced to show that 

 such ossification of extradigital cartilages really occurs. A 

 third theory which has been more generally accepted than the 

 ones just mentioned for the reason that a considerable amount 

 of evidence has been brought to its support by its author, is 

 Kiikenthal's theory of ** double epiphyses." These epiphyses 

 by retarded ossification become separated from the shaft of the 

 bone. Separated double epiphyses are commonly met with in 

 Cetacea and Kiikenthal mentions one case {Hyperoodon rostra- 

 tiis) in which they appear as separate bones even on the radius 

 and ulna. Once separated as distinct bones these epiphyses 

 would tend soon to become the same size as the bones from 

 which they separated, owing to similarity of function. Double 

 epiphyses in Mammalia would admit of as many as twelve 

 phalanges in a single digit, while as many as eighteen might 

 appear in the fourth digit of a reptile. These numbers are 

 rarely exceeded and only by certain toothed whales among the 

 mammals — Globiocephalus may have a maximum of seven- 

 teen — and by a few Ichthyosaurs, such as /. communis, among 

 the reptiles. To explain such cases Kiikenthal assumes the 

 formation again of double epiphyses in these secondary pha- 

 langes, and finally the development, by retarded ossification, of 

 a third set of phalanges. In this connection it is of impor- 

 tance to determine whether all of these groups of reptiles show- 

 ing hyperphalangy possessed epiphyses. These structures at 

 least appeared very early in some reptiles, and Kukenthal cites 

 the case of a Mesosauriis from the Karroo formation, which pos- 

 sesses double epiphyses. On the other hand it is worthy of 

 note that in the Chelonia, which according to Gadow do not 

 possess epiphyses, forms like Chelone and TJialassocJielys have de- 

 veloped remarkably long digits without any increase in the num- 

 ber of phalanges. 



Hyperdactyly. 



The presence of additional or supernumerary digits is known in 

 only two groups, the Ichthyosauria and Cetacea. In the former 

 group it occurs frequently, and as many as four extra digits or 

 nine in all are known in one species, Ichthyosaurus longimamis. 



