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THE AMERICAN NATURALIST. [Vol. XXXVII. 



British Museum and those at Edinburgh, but could find no 

 evidence that the folds are formed as extensions of the middle 

 layer of the shield. They appear to me to be covered by the 

 usual shell layers, but broken into small shell-like plates united 

 in such a manner as to allow some flexibility to the whole struc- 

 ture. They are not in my opinion specializations of the poste- 

 rior lateral margin of the cephalic shield, or of the cornua, but 

 true appendages attached to the under side of the head. The 

 large circular muscle scars seen on the inner surface of the dor- 

 sal shield (Fig. 5, m. m.) probably served for the attachment of 

 muscles moving these appendages. 



I cannot agree with Woodward and Lankester in regarding 

 these structures as opercula. for sections indicate that the sides 

 of the head were very thin, and that the most important cranial 

 organs were near the median line. Hence the so-called opercula, 

 standing so far back, and to one side, and when the animal was 

 at rest lying flat-wise against the bottom, were not in a position 

 to cover the gills, or to produce currents of water through them. 



Traquair regards the lappet-like Haps of the Coelolepidae as 



According to Dean, ('95, p. 69) the 

 : Pterichthys are " lateral head ang 



