1871.] 399 [Wilder. 



The layers of the shell do not extend around the whorl in Am- 

 monites, being wholly absent from the dorsal side. In Nautilus, 

 however, they do encompass the whorl, though the outer layer is very 

 thin on the dorsal side. The shell encompasses the ovisac in Ammo- 

 nites and Goniatites, but is very thin on the dorsal side of the em- 

 bryo, and very thick on the sides and abdomen. This at first led me 

 to suspect that the whole shell was deposited by the arms, as sug- 

 gested by Valenciennes, with regard to the deposition of the colored 

 stratum. 



But the structure of the shell in the walls of the ovisac is like that 

 of the adult, and shows that it, too, was deposited from within. All 

 the tetrabranchiate Cephalopods, therefore, are likely to prove to be 

 animals either wholly, or to a large extent, included in their shelly 

 coverings. 



Intermembral Homologies. By Burt G. Wilder, M.D. 



Continued from p. 339. 

 VI. SPECIAL PROBLEMS. 



Since it is probable that the telical antagonism of the membra with 

 some mammals must be eliminated from the discussion of their mor- 

 phical relations; and since the latest views upon the subject are based 

 upon the primitive condition and position of the membra in the 

 embryo ; and since they then do not indicate either syntropy or anti- 

 tropy, but are capable of interpretation upon either hypothesis ; and 

 since, finally, their adult condition points toward syntropy rather than 

 antitropy, so that the majority of anatomists are inclined to regard that 

 as their true and morphical relation; it is evident that we must not 

 merely remove the obvious objections to our way of thinking, but 

 must produce some positive evidence in its favor. 



This evidence consists in the establishment of the following proposi- 

 tions. 



1. The cephalic and caudal regions 1 of the body are comparable 

 with each other as are the right and left sides. 



iThe term region must here be taken to include all in front of, or behind, a mid- 

 dle point, and not merely the head or the tail. 



