INDICATIONS OP CRAWLING HABIT 



•J i 



possess color bands on one side. Silurian species of Gyrtoceras with short 

 curved shells and other breviconic forms have beon found by Barrande to 

 have been embellished with transverse zigzag color bands that passed 

 entirely around the conch. It is quite obvious that both color-markings 

 and form of conch are in both groups of shells in perfect harmony with 

 their mode of life — ^in Ortlioceras with a crawling habit, in which the 

 conch is dragged behind ; in the breviconic species of Cyrtoceras with a 

 crawling habit, in which the shell is carried obliquely or fairly upright 

 and, as suggested by the position of the 

 hyponomic sinus on the convex side, 

 with the apex pointing forward. 



The differentiation between dorsal 

 and ventral color-marking is lost in the 

 breviconic forms, both sides being 

 equally exposed to light and sight. 



6. FROM 8-ECONDARY DEPOSITS 



Certain of the straight-shelled forms 

 possess secondary deposits in the air- 

 chambers. It has been found that in 

 some of these, notably Actinoceras 

 tenuifilum (Hall) from the Black 

 River (Watertown) limestone of New 

 York and Canada, the secondary de- 

 posits which cover the inside of the 

 outer wall and the upper side of the 

 ,septa abut in thick rings against the 

 nummuloidal segments of the siphuncle 

 in such a fashion as to form a structure 

 that increases the resistance of the shell against being crushed. Such a 

 structure was of use only if the shell was dragged horizontally over the 

 ground. Forms like Gonioceras, with planoconvex section of the shell 

 and broad, low apertures, indicate already, by their form, their crawling 

 habit. 



Differentiation in Size of Sexes 



Another group of observations points to early differentiation in the size 

 of the sexes in some of the Paleozoic cephalopods. The Upper Utica 

 shale at Holland Patent, in New York, contains, in close association in 

 the same bed, three different forms of breviconic cephalopods, namely, a 

 larger and a smaller form with contracted apertures and a smaller form 



Figure 3. — Actinoceras tenuifllum 

 (Hall) 



Section through poi'tion of conch, 

 showing secondary organic deposits 

 of carbonate of lime (shaded) ; lower 

 part, section through siphuncle. Nat- 

 ural size. 



