g6 H. H. NEWMAN. 



correlation in the marginals Parker concludes that the abnor- 

 mality is in the anterior portion of the carapace, but an examination 

 of his figures fails to convince me that he has any criterion for 

 thus locating the point of suppression or recurrence of scutes, 

 for a loss anywhere in the marginals would necessitate a general 

 readjustment with reference to the costals. Yet the author con- 

 cludes that correlated abnormalities are likely to occur only in 

 the anterior portion of the carapace. 



To aid this hypothesis he adds a second one, based on a paper 

 by Harrison ('98), in which it is shown that in the frog larva the 

 ectoderm is proliferated chiefly at the anterior end and the meso- 

 derm chiefly at the posterior end. This would cause the ecto- 

 derm to slide back over the mesoderm. Parker concludes that 

 the ectodermal structures in the carapace have migrated away 

 from their mesodermal connections so that the posterior scutes 

 are far from their original plates, while in the anterior part of the 

 carapace the scutes are over the same plates that they originally 

 covered. 



This ingenious hypothesis loses its force when, after the ex- 

 amination of large numbers of abnormalities, it is found that nearly 

 all true correlations of plates and scutes occur at the posterior 

 end of the carapace. 



A reference to the tabulation of abnormalities will bring to 

 light the following facts : 



1. Specimens 2, 4, 6, 30, 45, 47, 61 and 62 show very precise 

 correlation of abnormalities in the marginals. All abnormalities 

 in the marginal scutes are correlated with similar abnormalities 

 in the plates. 



2. Specimens 5, 6 and 15 have abnormalities of both plates 

 and scutes of the costal series, but in no case are these strictly 

 correlated. In all three specimens the extra costal plates are 

 paired while the extra costal scutes occur on the right side only. 

 These specimens are abnormally long and the undue length may 

 be the common cause of both extra plates and scutes. 



3. No. 9 neural plate recurs in the same three specimens (5, 

 6 and I 5) and is associated in each case with one or more extra 

 neural scutes (Nos. 8 or 10). Examination shows that these 

 extra neural plates are irregular structures, are not associated 

 with the neural processes of vertebrae, and hence are to be con- 



