Diversity in the Scutes of Chelouia. 7 



upon Hay's view; but a view advanced by Gadow should be referred 

 to here, as it is not only the next in chronological sequence, but is 

 the first in which the atavistic interpretation was applied in a com- 

 prehensive way to the abnormalities of scutes. 



The two striking features of Gadow's paper are: (1) the attempt 

 to classify the variations and interpret them as reversions to ances- 

 tral conditions; and (2) the hypothetical explanation of these atav- 

 isms as stages in ontogeny or arrests of development. In his own 

 concise words, the abnormalities are viewed as "simply ontogenetic 

 stages, passing reminiscences of earlier phylogenetic conditions" 

 (Gadow, '05, p. 638). 



Originally, according to this author, there was a scute for each 

 dorsal plate. Thus, as there are eight transverse series of dorsal 

 plates, each series consisting of a median neural and a pair of lateral 

 costals, so there were originally at least eight transverse series of 

 dorsal scutes, scute and plate coinciding. But a process of reduc- 

 tion ensued. First, by the reduction of a pair of costals (probably 

 the second), the scutes of neural and costal series came to dovetail 

 into one another, and this dovetailing plan was subsequently retained 

 throughout all stages. Gradually the number of scutes was reduced 

 by the suppression — first, of the second costals, then of a neural; 

 then fifth costals, fifth neural, and finally by the fusion of the last 

 two costals; thus was attained the present typical condition of 

 Thalassoclielys with six neurals (including the nuchal), and five 

 pairs of costals. ^\lien turtles are found with more than this num- 

 ber of scutes the condition is to be regarded as "reminiscent" of 

 one of these phylogenetic stages. The order of suppression of scutes 

 given above is inferred from the relative frequency of recurrence of 

 "supernumerary" scutes in the several positions. Thus far, 

 Gadow's interpretation, though open to criticism, is interesting and 

 suggestive. Wlien he goes further, and regards these atavisms, when 

 found in adults, as instances of arrested development, or, when found 

 in younger turtles, as proper stages in ontogenetic recapitulation of 

 the phylogenetic stages, his position seems untenable on the basis of 

 any facts now in hand, as the writer has previously sho^vn ('05 a), 

 and as I^ewmann's observations also indicate (ISTewmann, '06. 

 p. 92). 



